In a food processor, whirl onion and garlic, pulsing on and off, until very finely chopped. Scrape into a bowl. Add half the mushrooms to processor and pulse until finely chopped (take care not to purée); scrape into bowl with onion mixture. Repeat to finely chop remaining mushrooms and scrape into bowl.
In a 10- to 12-inch frying pan over medium-high heat, melt butter. Add mushroom-onion mixture, thyme, and 1 teaspoon salt; stir often until liquid has evaporated, about 20 minutes.
Meanwhile, spread almonds in an 8-inch square pan. Bake in a 350° regular or convection oven, shaking pan occasionally, until nuts are golden beneath skins, about 10 minutes. Pour into food processor. Whirl until nuts are as finely ground as possible. Stir into warm mushroom mixture.
Pack into a small bowl (at least 2 1/2-cup capacity) or two 1-cup ramekins; cover with plastic wrap. Chill overnight to blend flavors. Bring to room temperature and serve in bowl or ramekins.
Cleaning mushrooms: Trim off tough or discolored bottoms of mushroom stems and any bruised spots or blemishes. (For shiitakes and oysters, remove the entire fibrous stem.) For firm mushrooms such as portabellas, wipe dirt off with a damp cloth or place in a colander, rinse thoroughly under cool running water, and pat dry with towels. For delicate mushrooms that have lots of places for dirt to hide, such as chanterelles and hedgehogs, submerge in a bowl of cool water and gently agitate with you hands to loose any particles. Drain, rinse carefully under running water, and gently pat dry with a towel.
Nutritional analysis per tablespoon.