Heat a large skillet over medium-high. Add olive oil and 1 tbsp butter until sizzling.

Add broccoli in a single layer. Do not stir for 2–3 min to brown.

Flip broccoli, add remaining butter, garlic, red pepper flakes, and ground mustard. Sauté 1–2 min.

Add water or broth, cover, and steam for 2 min until tender.

Remove lid, let moisture evaporate. Add lemon zest, juice, salt, and pepper.

Toss with Parmesan (optional) and let it melt slightly before serving.