Preheat your oven to 400°F. Line a large baking tray with tin foil.
Sprinkle the cherry tomatoes and sliced red onions evenly across the baking tray.
Cut small 1/2-inch slits in three places on both sides of the fish. Season the inside and outside with salt, pepper, and oregano.
Stuff the lemon slices and fresh dill inside each of the fish cavities.
Place the seasoned branzino on the lined baking tray and drizzle olive oil over the top.
Roast the branzino for 15 minutes or until the exterior is crispy and the fish is cooked through.
Remove the fish from the oven and serve immediately.