Heat 2 tablespoons olive oil in a 4 quart saucepan over medium heat. Add the diced onions and a pinch of salt; sweat the onions until translucent. Meanwhile, bring the water or stock to a simmer in a separate pot.
Purée the ripe tomatoes and celery in a food processor or a blender until smooth. Put the purée through a fine mesh strainer, keeping the liquid and discarding the solids.
Increase the heat in the pot with the onions and add the rice and another pinch of salt. Cook the rice for about 2 minutes to slightly toast.
Start adding the water or stock about 1 ½ cups at at a time. Stir frequently and add more liquid as soon as the previous is absorbed. Cook the rice for about 15 minutes, then stir in the tomato-celery juice. Continue cooking for about 5 more minutes, or until the rice is al dente.
Remove from heat and add the remaining olive oil, parmesan, lemon juice and basil. Season with salt + pepper to taste.
Mix the halved cherry tomatoes in a small bowl with 2 tablespoons olive oil, salt + pepper.
Plate the risotto and garnish with cherry tomatoes + grated parmesan.