Preheat a grill pan over medium-high heat. Brush the bread with 2 tablespoons EVOO and grill, turning once, until well marked and crisp, about 5 minutes. Rub both sides with the garlic. Cut into 1-inch cubes.
In a large bowl, combine the mustard, lemon juice and honey. Whisk in the remaining 3 tablespoons EVOO; season with salt and pepper. Add the arugula and radicchio and toss to coat. Divide among 4 plates and top each salad with the sardines and croutons.