- Heat oil in a large saucepan over medium heat and fry the chopped onions until soft and translucent, about 8-10 minutes. 
- Add the carrots and bell pepper (if using) to the pan and cook for another 3-4 minutes until slightly softened. 
- Stir in the minced garlic and ginger paste, cooking for 1 minute until fragrant. 
- Add all the spices (paprika, coriander, cumin, turmeric, fenugreek, and garam masala) and cook for 1 minute, stirring constantly to prevent burning. 
- Pour in enough water to just cover all the vegetables, bringing the mixture to a gentle boil. 
- Reduce heat and simmer uncovered for 45 minutes, stirring occasionally and adding more water if needed to prevent sticking. 
- Remove from heat and blend the mixture until completely smooth using an immersion blender or food processor. 
- Adjust consistency with water if needed - the sauce should have a milk-like consistency, neither too thick nor too thin.