Preheat the oven to 400°F.

Place the tomatoes, onion, and garlic on a rimmed half or large sheet pan. Drizzle with olive oil, sprinkle with garam masala, chilli powder, and 1½ teaspoons salt, toss to coat, and spread into a single layer with the cut side of the garlic head facing down. Roast on the bottom rack of the oven until the tomatoes get slumpy and start to brown in places and the onions soften, 32 to 40 minutes.

Meanwhile, prepare the toasts. Combine the grated cheeses, chopped cilantro, green onions, serrano pepper, garlic powder, and cumin seeds in a bowl and mix well. Place the halved baguette on a parchment or foil-lined sheet pan and divide the cheese mixture between the number of sliced bread you have, arranging it into an even layer. Set aside.

Remove the tomatoes and onions from the oven, leaving the oven on.

To finish the soup, if using a blender, transfer the roasted tomatoes and onions to the jar of the blender. Squeeze the head of garlic to release the roasted cloves into the blender, then discard the leftover head. Blend until smooth. Transfer the blended tomato mixture to a large pot. Add 3 cups chicken or vegetable stock to the blender, swish around to free any leftover blended tomatoes, add to the pot along with the remaining ½ teaspoon salt, and stir to combine. If the soup is a little too thick, add the remaining 1 cup of stock. Place the soup over medium heat, bring to a simmer, stir in the lemon juice, taste, and season with more salt or lemon juice, if necessary. Cover, reduce the heat to low, and keep warm while you finish the toasts.

Meanwhile, place the prepared toasts on the middle rack of the oven and bake until the cheese is melted and golden on top, 7 to 10 minutes.

To serve, ladle the soup into individual bowls, garnish with a drizzle of heavy cream, if desired, a sprinkle of chopped cilantro and a few cracks of black pepper, and enjoy with chilli-cheese toasts for dunking.