Steak - Sprinkle with some salt and pepper and tenderize with a fork. (Can be done 1 day ahead)
Onions / Brussels sprouts - (If prepping right before cooking, get oven heating before continuing with prep.) Prep as directed. Larger brussels sprouts should be quartered. Store separately. (Can be done up to 5 days ahead)
Caramelize onions - Heat up a frying pan over medium heat. Add oil (portion for steak) and then add onions to heated oil. Saute for ~10 minutes, until tender but still crunchy. If it looks like the onions or pan will burn at any point, just add a splash of water. Stir in with vinegar and maple syrup (portions for steak). Cook for another 2 minutes. (Can be done up to 5 days ahead)
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Spread brussels sprouts out onto a sheet pan. Cover tightly with foil and then bake for ~6 minutes.
Remove foil and toss brussels sprouts with oil, vinegar (portions for brussels sprouts), and mustard. Roast for another 8 to 10 minutes until crispy. Toss with maple syrup (portion for brussels sprouts) and salt, seasoning with more to taste.
Heat a skillet over medium-high heat. Add some cooking oil and then steak to heated pan. Cook steak, without moving it, until one side is well seared, ~5 minutes. Flip and cook on the other side for 4 minutes. Lower heat to medium, cover with foil, and cook for another 2 to 3 minutes for medium-rare; 4 to 7 minutes for medium; 8 to 10 minutes for well-done.
Set steak aside and let rest, covered, for 5 minutes.
If onions were caramelized ahead of time, reheat them in the skillet with some water or just heat in the microwave for ~1 minute.
Top steak with onions. Enjoy with brussels sprouts on the side.