In a large, lidded skillet, cook bacon over medium heat until crisp. Drain on a paper towel-lined plate and set aside. Pour off all but 2 tablespoons of bacon fat.
Season chicken thighs with salt and pepper. Place chicken in skillet skin side down, raise heat to medium-high and cook for about 6 minutes. Chicken skin should be nicely golden. Turn chicken and cook another 4 to 5 minutes. Transfer chicken to a plate.
If the chicken produced a good deal of fat in the skillet, pour off all but 2 tablespoons. reduce heat to medium and add butter, swirling to melt it. Add rosemary, garlic and red pepper flakes, if using, to the pan. Cook until fragrant, stirring constantly, about 45 seconds. Stir in lemon juice and broth, scraping up any browned bits.
Return chicken to pan, along with any accumulated juices. Crumble bacon over and around the chicken (to be honest, we split one strip of bacon—the three remaining strips were plenty). Cover the pan, reduce heat to medium-low and cook until chicken is cooked through, turning the chicken a couple of times to coat with the pan liquid, 10 minutes or more. An instant read thermometer should register at least 165ºF.
Plate chicken and spoon pan liquids and solids over it. Serve.