Preheat oven to 400° F
Line strips of bacon on a parchment-lined baking sheet. Cook bacon for 15-20 minutes, or until desired crispness. Transfer bacon to paper towel and drain all but 1 Tbsp of bacon grease from the pan. When cooled, crumble.
Trim asparagus ends. Spread onto baking sheet in a single layer – if using same one that bacon was on, remove used parchment.
Drizzle with the reserved 1 Tbsp bacon grease. Sprinkle with crumbled bacon and Parmesan cheese.
Roast until the asparagus is tender but still crisp and bright green and topping is starting to brown, about 15-20 minutes, depending on the thickness of the asparagus. Should be fork-tender.
Transfer to serving dish and squeeze lemon slices before serving. Serve immediately.