1. Heat oven to 400°F. Bring 2 cups water to boil in skillet over medium-high heat. Add carrots and cook 2 minutes; add peppers and asparagus, cook 2 minutes. Drain well.
2. Wipe skillet dry with paper towels; melt butter in skillet over medium heat. Cook onion 3 minutes; stir in spinach and cook 1 minute or until wilted. Remove from heat, stir in carrot mixture, bread crumbs, 1/4 cup mayonnaise, salt and pepper to taste. Let cool 10 minutes.
3. Meanwhile, whisk together sour cream, mustard and remaining 1 cup mayonnaise; cover and refrigerate until serving.
4. With a sharp knife, cut the loin filet down the middle lengthwise without cutting through. Open loin so it lies flat; cover with plastic wrap. Working from the center to edges, lightly pound loin with flat side of meat mallet or rolling pin to a rectangle with an even thickness throughout, about 1/2-inch thick.
5. Place the loin on a nonstick baking sheet; spoon vegetable mixture evenly over loin. Roll loin up starting from the longest side making sure the seam side is down (may secure with twine if desired, but not necessary). Roast about 45 minutes until internal temperature of the meat reaches 150°F. Let stand 10 minutes before cutting into 8 thick slices.
6. Serve 4 stuffed pork slices with half the sauce. Reserve remaining half of pork and half of sauce for the “Open-Faced Garlic Pork and Swiss Sandwiches” recipe.