Roasting the tomatoes makes them sweet and meaty; it also draws out much of the liquid so that you can assemble the sandwiches the night before without worrying about the focaccia becoming soggy.
Preheat oven to 350°.
Arrange the tomato halves, cut sides up, on a baking sheet. Drizzle with olive oil; sprinkle with 1 teaspoon thyme and black pepper. Bake at 350° for 45 minutes or until very tender. Cool slightly.
Combine remaining 1/2 teaspoon thyme, low-fat mayonnaise, olives, rind, and juice.
Spread cut sides of bread evenly with mayonnaise mixture. Divide tomato halves, Simple Roasted Leg of Lamb, onion, and artichokes evenly among 4 bread pieces. Top with remaining bread pieces. Wrap the sandwiches in foil or parchment paper; chill.