Cut pork diagonally into ¼-inch slices. Have remaining ingredients ready. Stir-fry pork in hot oil over high heat until no longer pink. Add sweet potatoes and toss until they begin to soften. Add peppers, snow peas, garlic, and gingerroot and continue stir-frying until peppers are crisp-tender. Combine soy sauce, water, and cornstarch in a small jar, shake well, and add to pan, stirring well. Cover pan, lower heat, and cook briefly, then uncover and stir in hoisin sauce. Serve with rice or noodles.