Combine ingredients: On a sheet pan, combine cherry tomatoes, red onion, olive oil, red pepper flakes, salt, and black pepper. Toss to coat everything evenly in the olive oil.
Roast: Roast for 20–25 minutes, or until the cherry tomatoes have burst and started to caramelize.
Serve: Serve warm or at room temperature as a side or topping for grains, salads, hummus, or fish.