Vegetarian Kofta Curry

1 tablespoon olive oil
1 tablespoon butter
1 teaspoon cumin seeds
3 green cardamom pods
2 whole cloves
1 cinnamon stick
1 bay leaf
1/2 onion, grated
1 (2 inch) piece ginger root, peeled and grated
1 clove garlic, crushed
1 (14.5 ounce) can diced tomatoes
1 teaspoon ground coriander
1/2 teaspoon ground turmeric
1/2 teaspoon ground cumin
1/2 teaspoon garam masala
1/2 teaspoon cayenne pepper (optional)
1/2 teaspoon salt, or to taste
1/4 cup whole milk yogurt
24 frozen meatless vegetable meatballs (such as IKEA®'s frozen vegetable balls)
1/4 cup hot water, or as needed

Heat olive oil and butter in a large saucepan over medium heat. Add cumin seeds, cardamom pods, whole cloves, cinnamon stick, and bay leaf. Cook until spices start to sputter, about 1 minute.
Stir in onion, ginger, and garlic.
Cook and stir until onion is softened and just starting to brown, about 5 minutes.
Stir in diced tomatoes, coriander, turmeric, cumin, garam masala, cayenne, and salt.
Bring mixture to a gentle simmer; add yogurt and stir well.
Add frozen vegetable balls.
Pour in hot water to thin sauce slightly to desired consistency.
Bring mixture back to a simmer and cook, covered, until sauce is thickened and vegetable balls are heated through, 10 to 15 minutes.