This may not be as mellow as you want--but you can reduce the vinegar and lemon and it may fit the bill. I LOVE this dressing! And it was from this very site!

http://www.thekitchn.com/go-to-salad-recipe-peanut-carrot-and-cabbage-slaw-172566

Hope you find something you like!

Add some dijon mustard to the dressing. Or, soak some chopped shallots in the vinegar (or lemon juice) for an hour or so, then add the olive oil & chives.

dijon mustard ought to do the trick! and a little bit of worcestershire

I made a sesame dressing to go with noodles but I think it would be great on salads because I tried it in a pasta salad with summer squash and mozzarella and it was so good.

Here is the link: http://sophieinthekitchen.blogspot.com/2012/06/sesame-noodles.html

try adding cheese! parmesan and feta are well-suited to salads. i find that sprinkling just a little bit of cheese over a salad ups the richness and saltiness without overpowering the veggies.

a little miso stirred into a dressing or vinaigrette works the same way (and the miso tahini dressing faith mentioned is absolutely lovely). sometimes, i just stir a spoonful of miso into my daily lemon/olive oil blend to add a little smooth-salty-savory goodness.

Grate about a half clove of garlic into the dressing you have, also add a touch of honey and a touch of dijon. The sweet of the honey makes all the difference.

If you're going more French, smash a few anchovies into a paste along with dijon and a bit of sugar as an abrasive, then add the dressing recipe you've already got going on (minus a bit of the salt to compensate for the anchovies.)

I like creamy dressings made with almonds instead of mayo. I have one that's like a garlicky aioli, with dijon and capers. It's nice on a slaw-type salad.

And one that's got basil, mint, lime, and tamari. Very summery!

I've been making miso-sesame dressing with a kick of ginger lately. It's so very good: mild and flavorful, light but just substantial enough to flavor every bite, and with the singing note of ginger in the background.

I don't measure, so the proportions are vague here:

a big spoonful of miso (I'm using red miso)
a palmful of whole sesame seeds
a splash of mirin
some grated ginger and a pinch of sugar (I often use a small spoonful of ginger jam, which I keep on hand, instead of fresh ginger, which I usually don't)
a few drops of sesame oil

I put all this in a jar and zzzzzzip it with the immersion blender, adding either a neutral oil or water to loosen up the mixture. Taste, add salt if necessary, and put it over annnnnnything. It's great with avocado salads, but this week I've been tossing blanched green beans and sliced cucumbers in miso-sesame dressing and serving them over mixed spinach and arugula. FANTASTIC.

Try this, but go easy on the anchovy paste (I love it, but since you want mellow...):

http://tastykitchen.com/recipes/salads/anchovy-vinaigrette/

Lemon juice and maybe a little anchovy paste. But seriously, adding a little lemon juice does remarkable things to brighten a salad.

A spoonful of miso or peanut butter (organic unsweetened) usually does the trick for any blah salad dressing. ;D

I had a creamy honey dijon at a catered event and would love to have that recipe!

I agree with the suggestion to add cheese, and I would add that if you find an olive-oil based dressing bland, you may be using the wrong olive oil. A good strong extra virgin olive oil should be anything but bland.

Maybe it's too bland because you don't dissolve the salt in the vinegar first. Try doing it (you stir it with a fork until the vinegar becomes cloudy), then you add the oil and you emulsify to get a real French vinaigrette (basic one is 1 spoon vinegar to 3 spoons oil). You might want to invest in good quality vinegar, non-balsamic as it's often too sweet. Another way to give a kick to your salad is to add some herbs (chives or chervil if you want something subtle, tarragon for something a bit more "aggressive"). Or just a few sliced scallions.

The vinegar has to be of excellent quality or you better replace it with fresh lemon juice. That's what I do most of the time. As Janet Brandt mentioned already, dissolve salt and all spices you may add in the lemon juice first before adding the oil with a whisk.

I most often add either chives or shallots or scallions (finely chopped) to a salad. I think, especially the sugar snaps would benefit from some fresh mint.

I like combining olive oil, Dijon mustard and a splash of red wine vinegar with lots of fresh cracked black pepper. Or for a little sweeter, I'll whisk some olive oil, lemon juice, sugar and catsup (kinda weird, but works!) together.