Place onions and olive oil into large skillet and toss a few times to coat onions. Place over medium-low heat. Allow to cook, uncovered, flipping occasionally, until onions begin to caramelize and brown in spots, about 25-30 minutes. Add garlic clove and cook 1 minute more. Remove from heat.
Place cashews, tofu, lemon juice and Worcestershire sauce into bowl of a food processor. Process until completely smooth, stopping to scrape down sides of bowl as needed. Season with salt and pepper and thin with water. Pulse a few times to blend.
Add onion mixture and pulse a few times, just until finely chopped and incorporated into mixture.
Transfer to a bowl and serve immediately or chill.