Heat the oil in a large nonstick skillet over medium-high heat. Add the onion, sugar and salt; cook, stirring occasionally, until onion is dark golden brown. Remove from pan. Add the spinach and 1 tablespoon water to the skillet; cook 2 minutes, or until spinach is just wilted. Remove from skillet and turn off heat. Spread a quarter of the goat cheese on each tortilla and top with spinach, tomato and onion; fold closed and press lightly. Heat the skillet and place two folded quesadillas in it; cook two minutes per side, or until golden brown and lightly crisp. Repeat.