Heat a large skillet over medium high heat and add the spinach and lemon juice (you may have to cook spinach in batches). Reduce heat to medium low and cover with a lid. Once spinach has wilted, remove lid, stir and remove from heat. Use a wooden spoon to squeeze out any water that collected while steaming.
On a large working surface lay out your flour tortillas. Start by spreading the goat cheese evenly between the four tortillas, followed by the red onion, beets, avocado, mozzarella, cooked spinach and fresh raspberries. Gently fold quesadilla closed and pressed lightly.
Heat large skillet over medium heat, adding 1-2 teaspoons of olive oil to the pan. Add folded quesadilla to your skillet, flipping once and cooking until each side is lightly crisp and golden brown (approximately 2 minutes each side).