For sauce: Prepare a fire for grilling. Oil grill.
Place tomatoes, onion and garlic on grill. Grill until skins of tomatoes are charred and tomatoes are softened. Onion should be browned on outside but still firm.
Remove vegetables from grill; peel and chop tomatoes, chop onions and peel and mince garlic.
Heat olive oil in a large non-stick skillet and add onion, sauteing until onions are translucent. Add garlic and cook for 1 minute. Add tomatoes, salt and pepper. Reduce heat to simmer and cook until sauce is reduced to a thick puree, about 30 minutes. Stir in basil and parsley.
For polenta: Butter a loaf pan and set aside.
In a heavy saucepan, over medium-high heat, bring water to a boil. Add salt and gradually pour in polenta, stirring constantly until thickened. Reduce heat and continue to cook and stir until mixture holds together and leaves a crust on sides of pan. Stir in butter until blended and transfer mixture to loaf pan, spreading evenly. Let cool, cover and refrigerate until firm, at least one hour.
Invert loaf pan onto a cutting board, removing polenta. Cut into pieces (squares, triangles or rectangles) and brush with oil.
Grill over medium-hot fire until browned on both sides.