Thinly slice half the onion. Blend the remaining half with the red bell pepper, tomatoes, garlic and optional hot pepper until reasonably smooth. Use the vegetable juices to help the blender; add a spoonful of water only if your machine needs it.

Heat the oil in a wide, deep skillet over medium heat. Add the sliced onion and cook for about 4 minutes, stirring, until softened. Add the ginger and tomato paste and stir for approximately 2 minutes without scorching the paste.

Carefully add the blended tomato-pepper mixture; it may splatter. Stir in the thyme, curry powder, bay leaf, broth, salt and black pepper. Bring to a simmer.

Cook uncovered or partially covered for approximately 20–25 minutes, stirring regularly, until the sauce is reduced and the vegetables no longer taste raw. Lower the heat if the base starts catching. Add a small splash of broth only if it becomes too dry before the vegetables cook.

While the sauce reduces, cook the spaghetti in a separate pot according to its package directions, stopping when it is just tender. Reserve 1/2 cup cooking water before draining.

Remove the bay leaf from the sauce. Taste and adjust the seasoning. Add the spaghetti and toss over low heat for about 1–2 minutes until coated. Add reserved water a tablespoon at a time only if necessary.

Serve promptly. Keep the heat gentle during the final toss so the sauce does not scorch against the bottom of the pan.