Preheat a grill or grill pan to medium high. Heat the cumin seeds, coriander seeds and 1/2 cup olive oil in a small skillet over medium-low heat until the seeds are toasted, about 3 minutes. Transfer to a liquid measuring cup. Transfer 3 tablespoons of the spiced oil and about half of the seeds to a small bowl; reserve for topping.
Put the bread cubes in a bowl, cover with water and let soak 2 minutes. Drain, squeeze dry and set aside.
Mince the garlic, then sprinkle with a pinch of salt and mash it into a paste with the flat side of a knife.
Toss the tomatoes in a bowl with the remaining 1 tablespoon olive oil and grill until charred, about 3 minutes per side. Transfer half each of the charred tomatoes, bread, garlic paste, sugar and vinegar to a blender; add 1/4 teaspoon salt and puree until smooth. With the motor running, add about half of the spiced oil from the measuring cup. Pour the mixture through a fine-mesh sieve into a bowl. Blend the remaining tomatoes, bread, garlic paste, sugar and vinegar with the other half of the spiced oil and 1/4 teaspoon salt, then strain into the bowl. Chill the soup at least 2 hours.
Dice the cucumber and bell pepper. Season the soup with salt and pepper. Ladle into bowls; drizzle with the reserved spiced oil and top with the diced vegetables.