Place croutons in a plastic bag, and crush to coarse crumbs with a rolling pin. (If using bread, grind in a food processor, and toast in a dry skillet until golden, 2 to 3 minutes, shaking skillet frequently to toast evenly.)
In a large skillet, melt 1 1/2 tablespoons butter over medium-low heat. Add cauliflower florets and 1/4 teaspoon salt; saute, 15 minutes. Add 1/4 cup water; cook until cauliflower is tender when pierced with a sharp knife, about 3 to 4 minutes.
In a large pot of boiling salted water, cook linguine until al dente, about 10 minutes. Drain pasta, reserving 1/2 cup pasta water.
In pasta pot, melt remaining 5 tablespoons butter over medium heat. Stir in sage and shallot; cook until butter is golden brown, 3 minutes. Add pasta, cauliflower, parsley, and Parmesan; season with salt and pepper. Toss to combine. Add reserved cooking water if pasta seems dry. To serve, sprinkle with breadcrumbs and more Parmesan.
To test pasta for doneness, toss a strand against a vertical stainless-steel surface (such as a refrigerator or backsplash). If it sticks, it's done.
To make this recipe as prepared on "The Martha Stewart Show," use 1/3 cup plain dried (coarse) breadcrumbs -- homemade or store-bought -- instead of the croutons or stale bread, and skip Step 1 of the instructions. Also, use 6 tablespoons of butter instead of 6 1/2.