Combine rapeseed oil, white wine, salt, pepper, basil, and lemon juice. Place turkey steaks in the marinade and turn to coat. Cover and refrigerate for 30 minutes.
Remove steaks from marinade and pat dry. Dredge in flour. Heat 2 tablespoons butter in a large skillet over medium-high heat. Fry steaks 2-3 minutes on each side until golden and cooked through (74°C / 165°F internal temperature). Transfer to a warm plate.
In the same skillet, add 1 tablespoon butter and reduce heat to medium. Add diced onion and cook until softened. Add mushrooms and oyster mushrooms, cook until tender. Season with salt and pepper, sprinkle with chopped parsley.
Arrange steaks on plates with the mushroom and onion mixture. Garnish each steak with a lemon slice and fresh parsley.