In an 8-inch non-stick skillet melt 1 tablespoon butter over moderate heat and in bowl toss with bread cubes. On a baking sheet toast bread cubes in middle of oven until pale golden and crisp, about 10 minutes, and transfer to a bowl.
Peel sweet potato and cut into 1/4-inch dice. In a steamer set over boiling water steam potato and onion until tender, about 4 minutes, and toss with croutons. Cool mixture and toss with goat cheese and rosemary. In a bowl whisk together eggs and salt and pepper to taste. In skillet heat 1/2 tablespoon butter over moderately high heat until foam subsides. Pour in half of eggs, tilting skillet to spread evenly over bottom. Cook omelet 1 minute, or until almost set, stirring top layer with back of fork and shaking skillet, letting any uncooked egg run underneath. Sprinkle half of omelet with half of crouton mixture and cook 1 minute more, or until set. Fold omelet over filling and transfer to a plate. Keep omelet warm while making another omelet with remaining 1/2 tablespoon butter, eggs, and crouton mixture in same manner.
