Mushroom Piccata

Name: Mushroom Piccata
Ingredients: 1 lemon; 4 Tbsp. extra-virgin olive oil, divided; 12 oz. mixed mushrooms (such as crimini, shiitake, and/or maitake), cut or torn into large pieces if needed; 1 tsp. all-purpose flour or cornstarch; Kosher salt; 1 large shallot or 1 small red onion, finely chopped; 3 garlic cloves, thinly sliced; ½ cup dry white wine; ½ cup Castelvetrano or other olives, crushed, pits removed; 1 Tbsp. drained capers; 2 Tbsp. unsalted butter, cut into pieces; Freshly ground black pepper; Parsley leaves with tender stems (for serving)