Salt the zucchini first: Lay the planks in a colander, sprinkle both sides with the 1 tsp kosher salt, and leave them in the sink for 15 minutes. Salt pulls water out of the cells through plain osmosis, and you’ll see it bead up on the surface. This matters more than it sounds: un-drained zucchini releases that water directly into the finished bowl, which thins the garlic cream from a sauce into a puddle. Dry planks also hit the grate and sear instead of steaming, so you get char marks rather than pale, floppy squash.
Season the steak while it waits: Pat the sirloin completely dry, rub it with 1 tbsp olive oil, then press on the salt, pepper, smoked paprika, and garlic powder. Let it sit at room temperature for the same 15 minutes the zucchini is draining a cold-from-the-fridge center takes longer to come up to temp and overcooks the outer half in the process.
Get the grill hot: Preheat to 450–500°F (230–260°C). Scrape the grates clean, then oil them with a folded paper towel held in tongs. Cold grates are how steak sticks and tears off half its crust.
Build the garlic cream: Melt the butter in a small saucepan over medium-low, add the minced garlic, and cook about 45 seconds, just until it smells sweet and nutty. Pull it the second it smells like anything approaching toast browned garlic turns bitter and there’s no rescuing it. Pour in the cream, bring it to a bare simmer, and let it reduce for 3 minutes until it coats the back of a spoon.
Finish the sauce off the heat: Take the pan off the burner completely, then whisk in the Parmesan, Dijon, salt, and lemon juice. Pro tip: residual heat is enough to melt the cheese, and it stops the proteins from tightening and going grainy a sauce that breaks on the stove almost never comes back. Cover and set aside.
Grill the steak: Lay it down and leave it alone for 4–5 minutes per side, until the center reads 130–135°F (54–57°C) for medium-rare. Flip once. Moving steak around the grate constantly keeps it from ever developing a proper crust.
Grill the zucchini while the steak rests: Blot the planks thoroughly with paper towels you want them visibly dry then toss with 1 tbsp olive oil and the black pepper. Grill 2–3 minutes per side until marked and just barely tender at the thickest part. Meanwhile, that steak needs 8–10 minutes of rest, and this is exactly how you spend it.
Slice and build: Cut the steak against the grain into ¼-inch strips. Divide the rice among four bowls, fan the steak over one side, the zucchini over the other, scatter the tomatoes and parsley, then spoon the garlic cream over everything. Serve right away, while the sauce is still warm and loose.
