I can handle mirepoix like flavor bases well when I make pasta sauces. But if I try something like making a flavor base for chicken pot roast the onions (which were sweated before adding the other ingridients) will burn while I reduce the liquids or when I simmer the chicken. Why?
In a simmered dish, there should be little risk of burning. Brown your chicken first, then remove from pan. Add veg. and use it to help release the fond. When the veg. begins to color, add liquid. Stir and scrape. Use more liquid in your simmer; that is, as it reduces, add more so that the veg does not burn. Return the chicken to pan, and be sure enough liquid remains to avoid burning.