Step 1 In a medium, heavy-bottom pot, cook the onions in the olive oil over medium heat until they start to turn a deep brown, about 45 minutes, stirring frequently.
Step 2 Stir in the garlic and thyme and cook just until aromatic, 1 to 2 minutes.
Step 3Add the tomatoes and water and bring to a gentle simmer. Cook until the tomatoes soften completely and begin to break apart, about 45 minutes, stirring occasionally. Remove from heat and cool slightly.
Step 4Puree the tomato mixture in a blender until smooth. (Be careful not to fill the blender more than 1/3 of the way or steam may force the liquid to fly out of the blender. And always cover the lid with a towel when blending hot liquids to prevent splashes and spills.)
Step 5Season to taste with salt and keep in a warm place until ready to serve.