Heat 1 tablespoon of oil in a skillet over medium heat until it shimmers.

Add the diced onions. Cook, stirring occasionally, until they are translucent and soft, about 3 to 4 minutes.

Add the ground beef and break it up with a spatula. Cook until it is browned and no longer pink, about 6 to 8 minutes.

Sprinkle the taco seasoning over the beef. Add salt and pepper to taste. Stir and let the seasoning cook briefly for about 30 seconds.

Remove the skillet from the heat and stir in the diced tomatoes and both shredded cheeses.

Wipe the skillet clean if needed and return it to medium-low heat. Place one tortilla in the pan.

Spoon half the beef-and-cheese mixture onto one half of the tortilla. Spread it evenly but leave a 1/2 inch border.

Fold the tortilla over the filling to form a half-moon, pressing gently with a spatula to seal.

Cook until the bottom is golden and crisp, about 2 to 3 minutes. Carefully flip and cook the other side until crisp, another 1 to 2 minutes.

Transfer to a cutting board and let rest for 1 minute. Slice into wedges. Repeat the assembly and toasting with the second tortilla.