This mustard sauce is meant to have some zing, but if you want less heat, swap smoked paprika for the cayenne.
Season steak all over with salt and pepper.
Whisk 1 grated garlic clove, ⅓ cup mustard, 1 Tbsp. vinegar, 1 tsp. honey, a couple pinches of cayenne, ⅓ cup oil, and 1 Tbsp. water in a medium bowl to combine; season spicy mustard with salt and pepper.
Heat a dry medium skillet, preferably cast iron, over medium-high. Rub steak all over with 1 Tbsp. oil and cook, turning every 2 minutes or so and making sure to get color on the fat cap, until medium-rare (an instant-read thermometer inserted into the center will register 120°), about 10 minutes. Transfer steak to a plate to rest. Pour off oil from skillet, leaving crispy bits behind.
Heat remaining 2 Tbsp. oil in same skillet over low. Add 4 sliced garlic cloves and all but about 2 Tbsp. scallions (save those for serving) and cook, stirring often, until translucent and softened, about 3 minutes.
Add peas and a splash of water and cook, stirring and mashing to break up slightly, until peas are tender, about 5 minutes. Add asparagus; season with salt and pepper. Cook, stirring often, until asparagus is just tender, about 5 minutes. Remove from heat.
Slice steak and shingle over vegetables in skillet.
Drizzle some mustard sauce over steak and top with reserved scallions. Serve with remaining mustard sauce alongside.