Remove the top sirloin and chicken breasts from the refrigerator 30 minutes before cooking to come closer to room temperature; pat both dry with paper towels.

Trim and season

Trim any excessive fat or silver skin from the sirloin. If chicken breasts are thick, butterfly or pound to an even 3/4-inch thickness. Season both sides of steak and chicken with 1 tsp kosher salt, 1/2 tsp black pepper, and 1/2 tsp garlic powder each (reserve remaining salt/pepper/garlic powder for later adjustments).

Make marinade/baste

Whisk together olive oil, soy sauce, Worcestershire sauce, lemon juice, brown sugar, minced garlic and chopped parsley in a small bowl. Reserve about 2 tablespoons of the mixture for basting during cooking.

Marinating

Place steak and chicken in separate shallow dishes or zip-top bags. Pour half of the remaining marinade over the steak and the other half over the chicken. Marinate at room temperature for 15–30 minutes, or refrigerate up to 2 hours. Remove from fridge 30 minutes before cooking if chilled.

Cook Steak

Sear

Heat a heavy skillet or cast-iron pan over medium-high heat until very hot. Add 1/2 tbsp vegetable oil and swirl. Remove steak from marinade (let excess drip off) and place in pan. Sear without moving 3–4 minutes until a brown crust forms.

Flip and baste

Flip steak and add 1 tbsp butter and 2 sprigs fresh thyme to the pan. Cook 3–4 minutes for medium-rare (internal temp 130–135°F / 54–57°C), longer if desired. Baste the steak continuously with melted butter and reserved marinade for the last minute.

Rest steak

Transfer steak to a cutting board, tent loosely with foil, and rest 8–10 minutes before slicing against the grain into thin slices.

Cook Chicken

Cook breasts

While the steak rests, wipe the skillet if needed and reheat over medium heat. Add the remaining 1/2 tbsp vegetable oil. Remove chicken from its marinade (shake off excess) and place in pan. Cook 5–6 minutes on the first side until golden.

Flip and finish

Flip chicken, add 1 tbsp butter and remaining thyme sprigs; cook another 4–6 minutes, basting with reserved marinade, until internal temperature reaches 165°F (74°C). Remove chicken to a cutting board and rest 5 minutes, then slice into 1/2-inch thick pieces.

Pan Sauce (optional)

Using the same skillet with fond, pour in beef broth and scrape up browned bits with a wooden spoon. Bring to a simmer and reduce about 3–4 minutes.

Whisk cornstarch into cold water to make a slurry. Whisk slurry into simmering broth and cook 1–2 minutes until sauce thickens slightly. Taste and season with remaining kosher salt and black pepper as needed. Swirl in remaining 1 tbsp butter for gloss and finish.

To Serve

Arrange sliced steak and sliced chicken on a warm platter or individual plates. Spoon pan sauce over or alongside. Garnish with chopped parsley if desired and serve with lemon wedges for brightening. Serve immediately.