Melt 2 tablespoons butter in the same skillet over medium heat. Add onion and cook until it starts to release its juices, about 2 minutes. Add mushrooms and ½ teaspoon salt; cook, stirring often, until tender and lightly browned, 2 to 4 minutes. Sprinkle with flour and cook, stirring constantly, for 1 to 2 minutes. Stir in tomato paste and cook for 1 minute.