Rinse the rice in a colander under cold water until it runs clear, picking out any little pieces of grit or debris. Shake off the excess water. Heat a pot over medium heat, and then add the rice, oil, garlic, saffron and cayenne pepper. Stir and toast the rice until the cayenne and saffron are fragrant, about 4 minutes. Shake the pot to level out the rice, and then add the chicken stock to fill about 1/2-inch over the rice. Bring to a boil, and then lower to a simmer and cover to cook for 15 minutes. Remove from the heat and let the rice stand another 5 minutes, covered. Fluff with a fork. Stir in the almond slivers and lemon zest, season with salt and pepper and serve.