On eight 10-inch skewers, alternately thread beef, zucchini, and red onion, piercing zucchini through the skin and leaving 1/4-inch between pieces. Add lemon wedges to ends of skewers. Brush all pieces with oil and season with salt and pepper.
For a gas or charcoal grill, place skewers on a grill rack directly over medium heat. Cover and grill 10 to 12 minutes minutes or until desired doneness, turning once.
Meanwhile, in a small bowl stir together the mayonnaise, sour cream, mustard, garlic, and tarragon. Serve sauce with skewers, squeezing grilled lemon wedges over all.