Preheat the oven to 450°.
In a 1-quart saucepan combine 3 tablespoons of the butter, the onions, celery, bell peppers, garlic, Worcestershire sauce, Magic Pepper Sauce®, and Magic Seasoning Blend®. Cook over high heat, stirring frequently and scraping the pan bottom well, for 5 minutes. Remove from the heat, let cool, and add to the meat mixture, along with ¼ cup of the milk. Mix well by hand and form into a 12 x 8-inch loaf and center in the pan (the meat loaf will not quite touch the sides of the pan). Bake until brown on top, about 30 minutes, and remove from the oven. Pour off the drippings, reserving 2 tablespoons plus 1½ teaspoons. Set the meat and reserved drippings aside.
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Meanwhile, boil the potatoes until fork tender. Drain, reserving 1 cup of the cooking water. Place the hot potatoes in a large mixing bowl with the remaining 1 stick butter, the remaining milk, the salt and pepper. Stir with a wooden spoon until broken up, then beat with a whisk (or electric mixer with a paddle) until creamy and velvety smooth. If the potatoes are not creamy enough, mix in a little of the reserved cooking water.
In a large skillet, preferably nonstick, combine the reserved drippings with the carrots, onions, and Magic Seasoning Blend®, and sauté over high heat, stirring frequently, for 1½ minutes. Add the zucchini and yellow squash, and continue cooking until the vegetables are noticeably brighter in color, about 3 to 4 minutes. Remove from the heat.
Raise the oven temperature to 525°.
Mound the undrained vegetables on top of the meat loaf, away from the edges. Layer all the mashed potatoes evenly over the top of the vegetables and top edges of the meat. Bake until brown on top, about 8 to 10 minutes, and serve immediately.