I much prefer to double the amount of vegetables in the recipe itself, something I discovered back in my own Weight Watchers days. By throwing in another onion, a few more chopped mushrooms, or an extra diced pepper, I could bulk up a dish without significantly changing the way it tasted. Or, more importantly, my sense of satisfaction in eating it.
This doubling trick works best in recipes that are already vegetable heavy, like hearty pasta sauces and soups. To adjust the seasonings, start by adding the spices called for in the recipe and then add more a little at a time until the dish tastes good to you.
Do you ever do this?