In a medium bowl, combine the diced tomato, finely chopped red onion, and fresh spinach. Add crumbled feta and shredded mozzarella, then sprinkle in dried oregano. Toss gently to mix.

Lay out your 8-inch flour tortillas on a clean surface. Take about 1/3 cup of the prepared filling and evenly distribute it onto one half of each tortilla. Carefully fold each tortilla in half, pressing down gently.

Place a large skillet over medium heat and add a light coat of olive oil. Allow it to heat for about 1-2 minutes.

Carefully place the folded quesadillas in the skillet. Cook for 3-4 minutes on one side until golden brown, then flip and cook for another 3-4 minutes.

Transfer the quesadillas to a cutting board and let them rest for 1 minute. Cut into wedges and serve warm.