In a 10- or 12-quart stockpot, bring water and salt to a boil. Add mustard greens, stirring to submerge and wilt. Boil greens for 5 minutes, uncovered. Stir and add pasta, return to a boil, and boil, uncovered, until pasta is firm to the bite, about 12-15 minutes (consult the pasta package for recommended cooking time for the variety you choose). Drain pasta and greens in a colander, return to stockpot and cover with lid.
While pasta and greens are boiling, heat olive oil in a skillet. Over low-medium heat, sauté anchovies and red pepper flakes (use larger amount if you like things spicier), pressing anchovies with the back of a wooden spoon so they melt into the oil. Add chopped garlic and sauté, stirring constantly, just until the garlic begins to color. This should take no more than a minute or two; if you let the garlic burn, the whole dish will taste bitter.
Pour garlic anchovy oil into the stockpot with the hot drained greens and pasta, stir to combine well, and season with freshly ground pepper. Warning: Taste the dish for salt, but with the salty anchovies and the salty Parmesan (as the finishing touch), you probably wont need additional salt.
Serve in wide, shallow pasta bowls, sprinkling 1 tablespoon grated Parmesan over top of each serving (if using).
Note: If you dont have penne, substitute any short pasta such as ziti, rigatoni, gemelli or corkscrews. This dish doesnt work that well with long strands of pasta such as spaghetti, linguine or fettuccine. Also, dont be afraid of the anchovies, which add a unique yet non-fishy background flavor to this main-dish pasta. Boiling the greens with the pasta gives the pasta more flavor and saves time.