Preheat oven to 425°F (220°C).
Slice the zucchini into 1/2-inch rounds. Cut the red bell pepper into 1-inch pieces.
Slice the yellow onion into thin wedges. Halve the button mushrooms if they are large, or leave them whole if small.
Line a baking sheet with parchment paper.
Toss zucchini, bell pepper, onion, and mushrooms with olive oil, Lemon Pepper spice blend, and a pinch of salt in a large bowl.
Season steaks generously with kosher salt on both sides. Press crushed black peppercorns into both sides of the steaks.