2. Slice off the top of the garlic bulb, just enough to barely expose the garlic inside each clove. Place the bulb in an ovenproof dish and drizzle with olive oil. Place the dish, uncovered, in the oven and cook the garlic until it is golden brown and soft, about 15 to 20 minutes. Let cool.
3. When the roasted garlic is cool enough to handle, squeeze the pulp from 6 of the cloves and mash with a mortar and pestle, or with a fork. (Store the remaining cloves in the refrigerator for up to 1 week.)
4. Put the garlic in a food processor and add the mustard, vinegar, salt and pepper and puree.
5. Slowly add the olive oil and stock through the feed tube, and puree until the vinaigrette is creamy. If it is too thick add a teaspoon of stock until the consistency is correct.
6. Add the shallots and tarragon. Adjust the salt and pepper to taste.
(The vinaigrette can be made in advance and stored in the refrigerator for up to 5 days.)
7. Gently toss the tomatoes in the vinaigrette.