Toss the halved tomatoes with the chopped onions, 2 Tablespoons of the olive oil and the minced garlic. Spread the seasoned tomatoes, cut side up, in a single layer on a rimmed sheet pan and bake them for 30 to 40 minutes. The tomatoes will have a shriveled appearance and are lightly brown around the edges. After roasting, turn off the oven, and leave the tomatoes in there for an additional 30 minutes, so that they cook in the existing heat.
Toss the roasted tomatoes with the remaining 2 Tablespoons olive oil, sage, rosemary, lemon zest, salt and pepper. Chill the tomatoes, with their marinade for at least 8 hours, or overnight to allow flavors to blend.