# 3 tablespoons butter
# 1 large white onion, cut into 1/2-inch pieces (about 2 1/2 cups)
# 18 ounces medium zucchini (about 4), trimmed, cut into 1/2-inch cubes
# 1 large red bell pepper, cut into 1/2-inch pieces (about 1 1/2 cups)
# 1 large orange bell pepper, cut into 1/2-inch pieces (about 1 1/2 cups)
# 1 10-ounce package frozen lima beans or shelled edamame, partially frozen
# 1 10-ounce package frozen corn kernels, partially frozen
# 1 tablespoon minced fresh marjoram
# Coarse kosher salt