Preheat oven to 500 degrees. In a roasting pan just large enough to hold plum tomatoes cut side down. Drizzle olive oil on bottom, and add garlic and half of the thyme and oregano. Top with cut tomatoes, cut side down, and roast in oven for 15 minutes. Remove the skins and process in food processor until roughly chopped. Steam open the clams, until just opened, with white wine. Strain the clam juice through cheese cloth to remove any sand. Reserve the juice and remove the clam meat. Make a sachet, all ingredients wrapped in cheesecloth, with parsley stems, oregano, thyme, bay leaves, and 1/2 head garlic. In a medium sauce pot sweat onion, celery, carrots, sachet, salt and pepper with olive oil for 5 minutes over medium heat. Add processed tomatoes and both clam juices. (Fresh and canned) Bring to a simmer and cook until the vegetables are tender. Cool off with sachet in mixture. Next to extract the entire flavor from the sachet, squeeze it well and discard. Season mixture with salt, pepper, and lemon juice. Cut the tender clams in small chunks. Heat the potatoes in a pan with olive oil and a bit of clam juice. Toss in the clams, some oregano, and parsley. Distribute garnish into hot bowls. Heat the broth and ladle over garnish at last minute.