Mesclun (MEHS-kluhn) is a salad or gourmet mix of tender small greens that include arugula, oak leaf, radicchio, sorrel, and a variety of greens depending on seasonal availability. Find it bagged or packaged in supermarket produce aisles.
Cook medium fresh beets, covered, in a medium saucepan in boiling water for 20 minutes. Add baby beets and cook 20 minutes more or until all beets are tender. Drain; cool slightly. Slip skins off beets. Quarter the medium beets; set aside.
For croutons, preheat oven to 300 degrees F. Tear bread slices into pieces (you should have about 4 cups).
Melt butter in a large skillet. Remove from heat; stir in garlic and, if desired, salt. Add the bread pieces, stirring until coated with butter mixture.
Arrange croutons in a single layer in a shallow baking pan. Bake, uncovered, for 10 minutes; turn croutons over. Bake 5 to 8 minutes more or until croutons are crisp. Cool completely.
Arrange greens and beets on eight individual salad plates. Top with croutons and drizzle with vinaigrette.