Put coriander and cumin in a spice grinder or clean coffee grinder and grind until fine. Transfer to a bowl. Put cashews in grinder and grind into a paste. Add to bowl and stir in turmeric, ginger, and garlic.
Grate tomato on the large holes of a box grater into another bowl, discarding skin. Set aside.
Heat oil in a large, straight-sided skillet over medium heat. Crack half the chiles open and add to pan along with onion. Cook, stirring, until onion softens, 2 to 3 minutes. Add peppers and continue cooking until vegetables begin to soften, 3 to 4 minutes. Add spice paste and 1/2 tsp. salt and cook, stirring often, until paste begins to fry, about 2 minutes. Add grated tomato, coconut milk, tamarind, brown sugar, and 3 tbsp. water; add 1 to 2 tbsp. more water if it seems thick. Reduce heat to medium-low and simmer 5 minutes, uncovered.
Tuck tuna pieces into sauce and gently simmer, covered, turning fish once, until almost cooked through, about 3 minutes. Stir in lime juice and season to taste with salt. Transfer to shallow bowls and top with herbs.
*Find tamarind concentrate at Indian markets, or add a little extra lime juice instead.