Step 1

Using a sharp knife, slice lemon into very thin rounds, (aim to get about 6–8 thin slices); remove seeds from rounds. Set remaining piece of lemon aside.

Step 2

Heat 3 Tbsp. oil in a large skillet over medium-high. Toss mushrooms with flour in a small bowl, then cook, tossing occasionally, until browned and crisp all over, 6–8 minutes. Season with salt and transfer to a large plate.

Step 3

Add remaining 1 Tbsp. oil to same skillet and reduce heat to medium. Cook shallot, stirring often, until softened, 2–3 minutes. Add garlic and cook, stirring often, until softened and beginning to turn golden around edges, about 4 minutes. Add wine, olives, capers, and lemon slices. Cook, swirling pan occasionally, until wine is mostly evaporated, about 2 minutes. Add butter and swirl skillet continuously until butter is melted and emulsified into sauce. Squeeze in 1 Tbsp. lemon juice from reserved lemon. Taste sauce and season generously with salt and a few grinds of pepper.

Step 4

Return mushrooms to skillet and cook, tossing and adding a splash of water to loosen sauce as needed, just until well coated.

Step 5

Top mushroom piccata with parsley just before serving.