Pat chicken dry. If breasts are thick, butterfly or pound to an even ~1/2–3/4-inch thickness. Season both sides with salt, pepper, onion powder, and smoked paprika.

In a small bowl, whisk together soy sauce, lemon juice, Worcestershire, olive oil, and minced garlic. Reserve about 2 tbsp of this for finishing the sauce; use the rest to lightly marinate chicken for 15–30 minutes if you have time (not required).

Heat a large skillet over medium-high heat. Add 2 tbsp butter and 1 tbsp olive oil. When shimmering, add chicken. Sear 3–4 minutes per side until golden. Remove chicken to a plate.

Reduce heat to medium. Add remaining 2 tbsp butter to the pan. Add sliced onion and cook 5–7 minutes, stirring occasionally, until softened and beginning to brown.

Add mushrooms and sauté another 4–5 minutes until they release their juices and those juices reduce slightly.

Pour the reserved soy-lemon mixture into the pan, scraping up browned bits. Let it bubble and reduce 1–2 minutes until glossy. Taste and adjust seasoning.

Return chicken to the pan, nesting it into the onions and mushrooms. Spoon sauce over each piece. Top each breast with a slice of Swiss cheese, cover the skillet, and let sit 2–3 minutes until cheese melts and chicken reaches 165°F.

Lightly toast buns (butter on cut sides) in a separate pan or under broiler for 1–2 minutes to avoid soggy buns.

Spread mayo on buns if desired, place smothered chicken on bottom bun, spoon extra onions and mushrooms on top, cover, and serve.