How do you cook onions and peppers so they get crispy?

While I am undergoing food allergy tests, I am on a strict diet. I am also a picky eater and picky about the texture of my food. I like crispy onions and peppers but they always turn out limp. I have tried cooking them on high heat on a grill pan and also broiled them once. My diet prohibits me from breading them or deep frying them. Any ideas on how I can make them crispy? Or any ideas on how to marinate them to give the veggies a different taste? Keep in mind, I cannot eat or cook in any oil; not even olive oil. I can use spray butter and any herbs and spices.