Rinse the rice in a strainer under running water until the water runs clear. (Catch the water in the saucepan and swish the strainer full of rice around in the water, then drain. 3 times usually does it.) This removes the starch off the outside of the rice and keeps the grains from clumping together.
Drain well, then put the rice in a saucepan with the water and vanilla and bring to a boil over high heat, uncovered, stirring several times. Reduce the heat to low, cover the saucepan, and simmer the rice without peeking, for 20 minutes.
While the rice is cooking, toast the slivered almonds in a single layer in a baking pan in the preheated oven for 10 minutes, or until they are fragrant and golden. This really brings out their flavour. Remove from the oven.
Chop the onion finely.
In a small skillet, heat the butter until it starts to turn light brown. Add the chopped onion and saute until the onion turns golden brown, with some of the edges turning medium brown. This caramelizes the natural sugars in the onion.
Add the raisins and toasted almonds to the onions. Turn off the heat and let sit in the skillet until the rice is cooked. When the rice is finished, reheat the onion mixture just until warm.
Turn the rice into a large bowl and add the onion-nut-raisin mixture, scraping in every last bit of flavourful browned butter with a spatula.
Sprinkle the salt and and a good grinding of pepper on top, and toss it all together well.