In a mixing bowl, combine diced large tomato, finely chopped red onion, and washed spinach. Add crumbled feta and shredded mozzarella cheese, sprinkling in dried oregano for an aromatic flavor boost. Pat the spinach dry to avoid a soggy quesadilla.

Lay out the 8-inch flour tortillas on a clean countertop. Take about 1/3 cup of your prepared filling and distribute it on one half of each tortilla. Carefully fold the tortillas over the filling, creating half-moon shapes.

Heat a non-stick skillet over medium heat and add a drizzle of olive oil. Once hot, place the folded quesadillas in the skillet, cooking for about 3-4 minutes until golden brown. Flip and cook the other side for another 3-4 minutes until cheese melts.

Transfer the quesadillas to a cutting board to rest for 1 minute, allowing the melted cheese to set slightly. Cut each quesadilla into halves or triangles and serve warm.