Step 1Heat a grill pan over medium-high heat until hot.
Step 2Lightly brush both sides of the bread slices with 1 tablespoon olive oil. Grill the bread until golden brown, about 2 minutes, then turn and grill the second side about 1 to 2 minutes. Rub the bread with the cut clove of garlic. Set aside.
Step 3For the vinaigrette, combine the balsamic vinegar and shallots. Whisk in 2 tablespoons olive oil. Toss the arugula with enough dressing to coat.
Step 4Divide the arugula onto grilled bread slices. Top each with three slices heirloom tomatoes. Sprinkle with fleur de sel, cracked black pepper, then torn basil and snipped chives. Drizzle over a little olive oil and balsamic vinegar. Top each sandwich with a slice of prosciutto.