Melt 2 tablespoons butter in a skillet over medium heat. Cook and stir the onions, parsley, and mushrooms in the melted butter until the onions are translucent and the mushrooms are giving off their juices, 7 to 10 minutes. Add the flour, salt, pepper, and lemon juice; cook until the flour is slightly browned, 2 to 3 minutes. Stir the sour cream into the mixture. Mix the green beans into the mixture. Pour the green bean mixture into a 9-inch square baking dish. Sprinkle the Cheddar cheese over the mixture. Stir the crackers and melted butter together in a small bowl until evenly mixed; spread evenly over the Cheddar cheese layer.
Bake in the preheated oven until bubbly and brown, 20 to 30 minutes.