Rinse the chicken pieces and pat dry thoroughly. Season with salt.
In a large Dutch oven or stockpot over medium-high heat, warm the olive oil until just smoking. Working in batches, sear the chicken until well browned on both sides, 8 to 10 minutes per batch.
Transfer the chicken to a platter and pour off all but 2 Tbsp of the accumulated fat. (If the bottom of the pot is scorched, discard the oil, wipe it clean, and add another 2 Tbsp oil.)
Turn the heat to medium and add the onion and celery. Cook, stirring frequently, until the onion is soft, 5 to 7 minutes.
Add the curry powder, 1/2 tsp black pepper, and tomato paste (if using) and cook, stirring constantly, until fragrant, about 30 seconds.
Add the tomatoes with their juices (if using), chicken stock, an raisins and bring to a simmer, scraping the bottom of the pot to remove all of the caramelized bits.
Nestle the chicken into the pot, cover, and turn the heat to medium-low. Simmer until chicken thighs are fork-tender, about 45 minutes.
Transfer the chicken to a platter, turn the heat to medium-high, and cook, stirring occasionally, until the sauce has reduced slightly, 5 to 7 minutes. Taste, adding more salt and pepper if necessary.
Stir the toasted almonds into the brown rice. Spoon on plates, and top with the chicken and sauce. Finish with the parsley. Serve immediately.