Toast the walnuts in a 350-degree oven until they release a fragrant odor, about 5-10 minutes.
Wash the greens and remove any coarse stems especially from collards and mustard greens.
Bring 3 inches of salted water to a boil.
Add the greens and boil for 5 minutes uncovered. Drain the greens and lay on a flat surface to cool; chop.
Heat oil in a large sauté pan. Add garlic and cook for 1 minute, stirring so the garlic does not burn. Add greens, walnuts, and salt to taste. Cook until greens are heated through.