Using a sharp serrated knife, cut off the tops of the tomatoes. DO NOT DISCARD. Before baking, we will place the tops back on the tomatoes.
Using a spoon, scoop out the insides of each tomato. Sprinkle a bit of salt inside each empty tomato and set each one upside down in a pan to drain. Put all pulp, seeds, and juice in a bowl. Using a hand blender, blend the pulp, seeds, and juice together until uniform. If desired, strain out the seeds. (I don't mind the seeds, so I leave them in.) The mixture will be very liquidy. Set aside.
In a saute pan, heat 1 Tbsp. olive oil and add onion. Saute for 2-3 minutes on medium heat until fragrant. Add dry rice, stir briefly and then add tomato pulp mixture and a pinch of salt. Stirring often, gently simmer uncovered until rice begins to soften (al dente) and most of the liquid is absorbed, about 10 minutes. If needed, add a spoonful of water or broth while cooking. Set aside. You don't need to cook the rice completely here because it will finish cooking in the oven when inside the tomato.
