1In large skillet over medium-high heat, melt 2 tablespoons of butter with olive oil. While heating, slice onions.
2Add onions to skillet. Add salt, pepper and sugar. Stir occasionally. As onions start to brown, as shown in picture, stir more frequently to avoid burning. Full caramelization will take about 20 minutes or so.
3While onions are cooking, place whole butternut squash in microwave for 2-3 minutes to make it easier to handle. Cut off top and bottom of the squash. If you have a sharp vegetable peeler, you can use it to remove the peel; otherwise use a knife. Cut the neck of the squash off from the rounded bottom. Then cut the rounded bottom in half and remove the seeds.
4Cut the squash into about 1" cubes. Melt butter, and add squash to skillet; salt and pepper the squash. Cook until squash is slightly browned and has softened (much like if you were making potatoes).
5Place a flour tortilla onto a griddle or skillet. Start building your quesadilla by adding as much or as little of the ingredients as you like. First, add the caramelized onion.
6Add some of the squash and top with crumbled goat cheese. Add a sprinkle of monterey jack cheese.
Fold tortilla over in half and brown both sides, allowing cheeses to melt.
7Serve with side of sour cream and slices of avocado.