Let's make it
Preheat oven to 400°; line rimmed baking pan with paper towel. Place tomato slices in single layer on prepared pan; sprinkle with ¼ teaspoon salt and let stand 10 minutes.
In large skillet, heat oil over medium-high heat. Add shallot; cook 2 minutes or until tender, stirring occasionally. Add mushrooms; cook 4 minutes or until moisture is released, stirring occasionally. Add garlic; cook 30 seconds or until fragrant, stirring occasionally.
In large bowl, stir egg, cheese, mayonnaise, basil, mustard, pepper, remaining ¼ teaspoon salt and mushroom mixture. Makes about 2 cups.
In bottom of pie crust, layer 2/3 cup egg mixture and half of the tomatoes; repeat layers and top with remaining 2/3 cup egg mixture.
Bake pie 40 minutes or until top is golden brown and internal temperature reaches 160°; cool 30 minutes on wire rack. Cut pie into 8 pieces; serve sprinkled with onions.
Approximate nutritional values per serving (1 piece): 276 Calories, 21g Fat (6g Saturated), 36mg Cholesterol, 566mg Sodium, 16g Carbohydrates, 1g Fiber, 4g Sugars, 7g Protein