Heat oil in a medium skillet over medium-high heat; add onions and cook until they begin to soften.
Add zucchini, corn, jalapeno, a squirt of lemon or lime juice and seasonings to taste; Lower heat to medium and cook until veggies are tender.
Place tortilla in a large skillet over low heat; place cheese and tomato slices on half the tortilla, top with zucchini mixture, and fold the empty side of the tortilla over it, pressing down lightly with a spatula.
When the cheese begins to melt and the tortilla starts to brown, flip and brown the other side as well.
Slice into wedges and serve with salsa or sour cream.
**For 2 quesadillas (or a mega-quesadilla), double the filling ingredients, spread them on the entire tortilla and place another tortilla on top -- flipping is a little trickier, but otherwise the method is the same).