Heat oil in a large nonstick skillet or Dutch oven over medium-high heat. Add onion and cook, stirring, until beginning to soften, about 1 minute. Add garlic and cook, stirring, until fragrant, about 30 seconds more. Add spinach and cook, stirring, until just wilted, about 2 minutes. Remove from heat; stir in bacon and sprinkle cheese on top. Serve immediately.
Ingredient note: The sturdier texture of mature spinach stands up better to sauteing than baby spinach and it's a more economical choice. We prefer to serve baby spinach raw.