Rinse 2 cups rice under cold water until water runs clear; drain and set aside.
Roughly chop tomatoes, red bell pepper, and half the onion; remove seeds from the pepper if desired.
Blend tomatoes, pepper, and half the onion into a smooth purée; set aside.
Heat 3–4 tbsp oil in a large heavy pot over medium heat until shimmering.
Finely chop remaining onion and sauté until translucent and slightly golden.
Stir in 3 tbsp tomato paste and fry 2–3 minutes to remove raw taste and deepen color.
Add blended tomato mixture to the pot; simmer 10–12 minutes, stirring occasionally, until reduced and fragrant.
Season with thyme, curry powder, smoked paprika, bay leaves, salt, pepper and optional bouillon; simmer 5 more minutes.
Pour in hot stock (start with 2 cups) and bring sauce to a gentle boil; adjust seasoning.
Add drained rice in an even layer; do not stir vigorously—gently level the rice.
Reduce heat to low, cover tightly and cook 18–22 minutes until rice absorbs liquid and is nearly done.
If rice is still firm, add up to ½ cup more hot stock, cover and steam 5–8 minutes until tender.
Remove from heat and let rest 5–10 minutes with lid on to finish steaming.
Fluff rice gently with a fork, fold in cooked chicken or vegetables if using, and serve hot.