Heat oil in a large skillet over medium-high heat. Add garlic cloves; cook until golden brown, about 1 minute. Remove with a slotted spoon and discard. Add guanciale; cook and stir until crisp and golden, about 4 minutes. Add onion and red pepper flakes; cook and stir until onion is translucent, about 3 minutes. Stir in tomatoes, salt, and black pepper. Simmer tomato sauce until flavors combine, about 10 minutes.