Spray a large nonstick frying pan with nonstick cooking spray. Add olive oil and heat over medium flame.

Add the onions and sauté until golden, about 6–8 minutes. If the onions begin to stick, add a little water.

Add the mushrooms and garlic and cook for 6–8 minutes, stirring occasionally, until nicely browned. Remove the pan from the flame and cool slightly.

In a food processor fitted with the S blade, process walnuts until finely ground, about 10–12 seconds. Add the onion/mushroom mixture, eggs, salt, and pepper. Process with several quick pulses, until just combined.

Transfer to a container, cover, and refrigerate until ready to serve.

Scoop into individual Bibb lettuce leaves or radicchio lettuce cups, or on salad greens.