Slice onions into 1” slices; place on a large plate and set aside.
Slice each bell pepper lengthwise into quarters; remove seeds. Place peppers in a large bowl.
Remove peel and pit from avocado; slice into ½” slices; place in bowl with peppers.
Slice each tomato in half and remove seeds and pulp; place tomatoes into bowl.
Using a small bowl, whisk together olive oil, salt, smoked paprika, garlic powder, onion powder and black pepper. Brush some of oil mixture onto both sides of onions. Pour remaining oil mixture into bowl with other veggies; toss until well coated.
Transfer onions and veggies onto grill. Grill for 4-6 minutes, flipping as needed. (Note: The tomatoes and avocado will only need 3-4 minutes. The onions and peppers will need a little longer.)
Remove veggies from grill and transfer to a large cutting board; let cool for 10 minutes.
Using a sharp knife, chop veggies into small pieces; set aside.
Using a medium bowl, mix cheeses together.
Lay tortillas out on countertop. Divide chopped veggies mixture into quesadillas. Divide shredded cheeses evenly on top of veggies. Sprinkle green onions evenly on top of cheese. (Tip: Spread fillings onto half of the tortilla so that it’s easier to fold tortilla over.)
Fold each tortilla over to create a half-moon quesadilla.
Place quesadillas on grill. Grill for 2-3 minutes per side, or until cheeses have totally melted and tortillas have just begun to turn crispy.
Remove quesadillas from grill and slice each into 4 wedges.
{Optional} Serve with sour cream, cilantro and salsa.