Heat oven to 425 degrees. Place broccoli and cauliflower together on one large rimmed baking sheet or pan, and beets and turnips on another. Place squash and 1 bunch scallions on individual pans. Toss all vegetables with olive oil, salt and pepper until well coated. Roast scallions 12 to 15 minutes; broccoli and cauliflower 20 to 30 minutes; squash 30 to 40 minutes; and beets and turnips 40 to 50 minutes. Toss all periodically while roasting.
Step 2
Combine garlic, lemon juice and salt in a blender or food processor and let sit a minute or two. Add eggs and blend until combined. With the blender running, slowly add the olive oil in a thin, steady stream. You can use a mortar and pestle if you prefer. Taste for seasoning and add more salt and lemon juice if needed.
Step 3
Arrange roasted vegetables on platter and sprinkle with pomegranate seeds if using. Serve with the aioli on the side.