In a large saucepan or Dutch oven, cook the bacon in the olive oil over medium heat, stirring occasionally, until rendered but not crisp, about 5 minutes. Add the celery, onion, garlic and a generous pinch each of salt and pepper. Cook, stirring occasionally, until softened, about 8 minutes. Sprinkle the flour evenly on top and cook, stirring, until the vegetables are evenly coated, and a film starts to form on the bottom of the pan, about 2 minutes.