1. To roast garlic, preheat oven to 375°F. Cut off the top ½ inch of the garlic heads, revealing the cloves. Place them in a baking dish to fit; cover each head with ½ teaspoon of extra-virgin olive oil. Cover with heavy foil; crimp tightly and bake until softened and caramelized, 50 to 60 minutes. You can do this up to three days in advance.
2. Remove from oven to cool, then squeeze the cloves out into a bowl; discard the skins.
3. To roast the scallions, lay them on a parchment-lined sheet pan, rub with a scant amount of extra-virgin olive oil, and sprinkle with a pinch of sea salt. Roast at 375°F until just colored around the edges, about 10 minutes. Cool, discard the root end, and chop coarsely.
4. While scallions are roasting, toast the onion in a dry skillet over low to medium heat, shaking until lightly golden, about 2 minutes. Cool.
5. Assemble dip: In a medium-size bowl, add the roasted garlic cloves and roasted scallions. Take 2 tablespoons of toasted dry onions and crumble them with your fingers to break up a bit. Add the rest of the dried onions, parsley, celery seed, and nutritional yeast. Mash all with a fork to combine. Add the sour cream or silken tofu and blend well. Taste for salt.
Cover and refrigerate for at least an hour to blend the flavors. It will stay fresh for at least one week.