Toast slivered almonds in a small skillet over moderately high heat until golden, about 2 minutes. Let cool.

Season pork medallions with salt and pepper, then coat in flour.

Melt 2 tablespoons of butter in olive oil in a large skillet.

Add medallions and cook over moderately high heat until lightly browned on both sides and just cooked through, about 4 minutes.

Transfer pork to a plate.

Add onion to the skillet and cook over moderate heat until softened and just beginning to brown, about 4 minutes.

Add garlic and cook until fragrant, about 1 minute.

Add Riesling, stock, raisins, vinegar, and thyme and bring to a boil.

Cook over high heat, scraping up any browned bits from the bottom of the pan, until the liquid has reduced slightly, about 2 minutes.

Return the meat to the skillet and simmer until hot, about 1 minute.

Transfer pork to a platter.

Add the remaining 1 1/2 tablespoons of butter to the sauce and cook over moderate heat, swirling, just until the butter is melted.

Pour the sauce over the pork, garnish with the toasted almonds, and serve.