Prep the steak: Pat the steak cubes dry with paper towels.
Season with 1/2 teaspoon salt and black pepper. Let them sit at room temperature for 10 minutes while you prep the broccoli.
Make the glaze: In a small bowl, whisk together balsamic vinegar, soy sauce, honey, minced garlic, and a pinch of red pepper flakes if using. If you prefer a thicker sauce, stir in the cornstarch until smooth.
Cook the broccoli: Heat a large skillet over medium-high.
Add 1 tablespoon olive oil. Toss in the broccoli and 1/4 teaspoon salt, and cook for 2–3 minutes, stirring, until bright green with some char. Add 1/4 cup water, cover, and steam for 2–3 more minutes, until crisp-tender.
Transfer broccoli to a plate.
Sear the steak bites: Increase heat to high. Add a little more oil if the pan looks dry. Arrange steak cubes in a single layer (cook in batches to avoid crowding).
Sear 1–2 minutes per side until browned. Don’t move them too much—you want a good crust. Transfer to a bowl and repeat with remaining steak.
Butter baste: Reduce heat to medium.
Add 1 tablespoon butter to the empty pan. Once melted, return all the steak to the skillet and toss for 30 seconds to coat and finish to your preferred doneness. Remove steak to a plate to rest.
Deglaze and sauce: Pour the balsamic mixture into the hot pan, scraping up any browned bits.
Simmer for 1–2 minutes. If you didn’t use cornstarch, add a splash of water as needed and simmer until slightly syrupy.
Bring it together: Return broccoli and steak to the skillet. Toss to coat in the glaze.
Add the remaining 1 tablespoon butter and stir until glossy. Taste and adjust salt and pepper.
Serve: Garnish with chopped parsley or chives. Serve over rice, mashed potatoes, noodles, or cauliflower rice, spooning extra sauce over the top.