In a bowl, combine butter and pepper. Add bread cubes and toss to coat. Arrange in a single layer in an ungreased 15x10x1-in. baking pan. Bake at 400° until golden brown, 8-10 minutes, stirring occasionally. Cool.
In each of four 1-qt. wide-mouth canning jars, divide and layer ingredients in the following order: dressing, pastrami, sauerkraut, tomato, green onions, cheese and salad greens. Cover and refrigerate until serving. Divide croutons among 4-oz. glass jars or other small containers; cover. To serve, transfer salads and croutons into bowls; toss to combine.