What if you used charred/roasted peppers and sautéed with lots of onions and carrots and add some vegetable stock?

Is this for a particular recipe?

Roasted eggplant, mushrooms, zucchini, and bell peppers would also work nicely.

I agree on the roasted red peppers although I don't think they harmonize well with pesto.

You could do a roasted red pepper and butternut squash sauce.

Hi! Butternut squash makes a good sauce for substituting tomatoes, I have used it for pizza, pasta... If you are not allergic to peppers those are a tasty option :) good luck!

Rhubarb makes a fun and summery pasta sauce on its own or added to roasted bell peppers. Carbonara is always one of my favorite pasta toppings as well.

I recently did a caramelized carrot sauce based off of Nathan Myhrvold's soup method: http://www.foodandwine.... I didn't add as much of the juice and folded in lots of fresh herbs. I also made it once with little spinach and cheese meatballs. The depth that you get from the butter and caramelization is incredible.

I'd go for pumpkin puree or carrots with a splash of red wine vinegar or balsamic (to make up for the acidity you'd normally get with tomatoes). Roasted red peppers, while similar in color, are a very dominating flavor, in my opinion; they aren't very versatile unless you're going specifically for that flavor.

Also, often times, people with tomato allergies are only reactive to raw tomatoes; but once they are cooked, the allergen that causes the reaction is no longer there. Just a side note (that you might already know...?).

Macrobiotic sources should have a tomato-style pasta sauce recipe. I have one in Whole World Cookbook (1984) made with carrots and a beet, but I've never tried it.

The butternut squash puree is a good one, especially if you top each serving with sage leaves fried to crisp in butter with a dribble of the sage butter sauce over the top.