2. Slice the tips off the garlic heads, exposing the cloves. Place the heads in a small baking dish. Pour 2 tablespoons oil over them and sprinkle with ¼ teaspoon each salt and pepper. Roast until the garlic feels soft when you squeeze the bulb, 20 to 40 minutes, depending on size.
3. When cool enough to handle, gently squeeze garlic cloves from the skins into the dish (discard skins). Add chives and chopped garlic. Swirl in lemon juice.
4. Peel the tough outer layer of the bottom half of asparagus stalks, if desired. Place the asparagus on a rimmed baking sheet; drizzle with the remaining 1 tablespoon oil and sprinkle with lemon zest and ¼ teaspoon each salt and pepper. Roast, shaking the pan halfway through, until the asparagus is just tender, 10 to 20 minutes. Let stand for 5 minutes.
5. Divide the warm asparagus among 6 plates. Top each portion with about 2 tablespoons roasted garlic vinaigrette and 1 generous tablespoon walnuts.