In food processor, pulse mushrooms in 3 batches. (Don’t process too finely; mixture should resemble fine gravel.) Transfer mushrooms to bowl; add half of lemon juice. Warm 2 tablespoons butter in non-stick sauté pan over low heat; add mushrooms and cook, stirring frequently, until all liquid has completely absorbed, about 35 minutes. Remove from heat and cool. Season with 1 teaspoon salt and 1 teaspoon black pepper.
In another sauté pan, place bacon and cook over medium heat until golden and beginning to crisp, about 5–6 minutes. Remove bacon to paper towel to drain. Add onion to same pan and sauté over medium heat for 5 minutes until onions are soft, stirring often. Remove from heat and cool.
Preheat oven to 350°F. Place chicken on flat surface and season both sides of each piece, using 1 teaspoon salt and 1 teaspoon black pepper. Allow to sit for 10 minutes.
In bowl of food processor, place garlic, 1 stick of butter, herbs, lemon zest and remaining lemon juice; process until smooth. Transfer to large bowl. Combine with mushrooms, bacon and onion; blend well. Season with remaining half teaspoon salt and half teaspoon black pepper.
Gently run finger under skin of chicken. Divide mushroom-herb mixture into 4 equal portions; stuff under skin of each piece of chicken. Place in roasting pan and roast in oven 40–50 minutes, until skin is deep golden brown.