Preheat oven to 425 degrees. Wrap the garlic cloves lightly in foil and set on a baking sheet. Toss the onions with 1 tablespoon oil and arrange in a single layer on a baking sheet. Roast for about 1 hour, or until garlic is softened, and onions are soft and lightly charred all over. Turn onions halfway through so they cook evenly.
Remove pan from oven and let cool.
When garlic is cool enough to handle, squeeze cloves from their peels, and place in the bowl of a food processor, along with onions, oil, vinegar, lemon juice and honey. Process until dressing is smooth.
Season with salt and pepper, and add water if a thinner consistency is desired.
Pulse a few more times to combine.