Step 1

Melt 6 tablespoons butter with oil in heavy large skillet over high heat. Add mushrooms; sprinkle with salt and pepper. Sauté until mushrooms are tender and beginning to color, 5 minutes. Stir in garlic, then Marsala, 1 tablespoon lemon juice, tomato paste, and lemon peel. Simmer until Marsala has evaporated, about 1 minute. Cool.

Step 2

Transfer mushroom mixture to processor; add parsley and 10 tablespoons butter. Process until mushrooms are finely chopped. Season to taste with salt, pepper, and more lemon juice, if desired. Transfer to small bowl. Cover and refrigerate overnight to allow flavors to develop. DO AHEAD Can be made 1 week ahead. Keep refrigerated. Serve rillettes with toasted baguette slices.