1. Whisk together garlic, oil, vinegar, salt (in moderation) and ground pepper. Remove the stems from the mushrooms and discard. Slice the onion into 4 1/4-inch slices. Generously brush mixture over both sides of mushrooms and onions. Reserve any remaining mixture.
2. Heat a gas or charcoal grill to medium hot. Grill 8-10 minutes, flipping once, or until softened and cooked through. While vegetables grill, brush bread slices on both sides with remaining garlic, oil and vinegar mixture and grill, turning once, 2-3 minutes, or until toasted.
3. Slice mushrooms. Top each toasted bread slice with 1/4 of the arugula, mushroom slices, grilled onions, dip and crumbled cheese. Serve immediately, passing extra sweet potato dip, if desired.
Per serving: 297 calories, 18g fat, 4g saturated fat, 17mg cholesterol, 441mg sodium, 27g carbohydrate, 4g fiber, 7g sugar, 9g protein