In a large skillet, heat 2 tablespoons of oil over medium-high heat. Once the oil is shimmering, add the trimmed asparagus and sauté for about 3-5 minutes, tossing occasionally, until bright green and tender-crisp. Remove the asparagus from the skillet, setting it aside on a plate while you prepare the chicken.

In the same skillet, reduce the heat to medium and melt 3 tablespoons of butter. Season the butterflied chicken with salt and pepper before adding it to the pan. Cook for 3-5 minutes on each side until the chicken is golden brown and cooked through, reaching an internal temperature of 165°F. Once done, transfer the chicken to the plate with the asparagus.

In the same skillet, add the chopped garlic and a pinch of red pepper flakes if using. Sauté for about 1 minute until the garlic is fragrant and golden, being careful not to let it burn.

Pour in 1/2 cup of chicken broth, scraping the bottom of the skillet to release any tasty browned bits. Turn up the heat to bring the mixture to a gentle boil, then reduce the heat and simmer for about 5 minutes.

Stir in the lemon juice and zest along with the chopped fresh dill. Return the chicken and asparagus to the skillet, coating everything evenly with the sauce. Allow the dish to heat through for another minute before serving warm.