1. Place about 2 cups mushrooms in a food processor, and pulse 12 times or until finely chopped, scraping sides of bowl occasionally. Place chopped mushrooms in a large bowl. Repeat procedure in batches with the remaining mushrooms.
2. Heat a large nonstick skillet over medium-high heat. Add oil to pan; swirl to coat. Add onion, leek, and garlic to pan; sauté 3 minutes or until tender. Stir in mushrooms; sauté 30 minutes or until liquid evaporates, stirring occasionally. Remove from heat; stir in parsley, chives, salt, and pepper. Cool completely.