Heat olive oil in a large saucepan over medium-high heat. Cook and stir bacon, onion, and garlic in hot oil until bacon and onion are thoroughly browned, 5 to 7 minutes. Reduce heat to medium-low. Stir diced tomatoes and basil through the bacon mixture; cook at a simmer for 5 to 10 minutes.
Stir cream of mushroom soup through the tomato mixture and about 1/2 cup milk. Stirring constantly, increase heat to medium and slowly stir Parmesan cheese into the mixture. Gradually stir 1/2 cup milk into the mixture until it is creamy enough for you preference. Cook and stir until hot, about 5 minutes. Stir parsley into the mixture just before serving.