Stir together 1/2 cup mustard, 1/2 cup honey, 2 tablespoons lemon juice, garlic, salt, and 2 teaspoons pepper in a large shallow dish or heavy-duty zip-top plastic bag; add pork, turning to coat. Cover or seal, and chill 3 hours. Place pork on a lightly greased rack in an aluminum foil-lined roasting pan.
Broil 6 inches from heat 5 minutes; reduce oven temperature to 425°, and bake 15 minutes. Cover loosely with foil, and bake 15 more minutes or until a meat thermometer inserted into thickest portion registers 160°.
Stir together remaining 1/2 cup mustard, remaining 1/4 cup honey, remaining 1 tablespoon lemon juice, remaining 1/2 teaspoon pepper, and mayonnaise. Serve mixture with pork.