Put the walnuts on a plate and toast them in the microwave. Check after 1 minute, and then every 30 seconds. It will take about 2 minutes to get them lightly toasted. Set aside.

Place a wide skillet over medium heat and add the olive oil. When barely hot, add the onions and garlic and sauté until the onions are golden. Add all the remaining ingredients—mushrooms, parsley, rosemary, salt and pepper—and sauté until the mushrooms are cooked and reduced in size, about 5–8 minutes. If the mushrooms are sticking to the pan add a small splash of water to help release them. As the mushrooms cook they will release their own liquid.

Put mushroom mixture and walnuts in a food processor. Pulse, stopping to scrape the sides as needed, until you reach a texture you like. I like to leave mine with a bit of texture and color variation. Taste and adjust seasonings. Press into a container for serving, using a spatula to smooth the top. Chill.