1. Heat up the grill on high for direct heat.
2. Soak your plank for at least 1 hour (or longer, you cannot oversoak it) to avoid it from catching fire. Make sure it is weighed down with a plate and fully submerged and flip it half way. Tip, You can also soak your plank in apple juice, beer, or wine to add more flavor to the fish.
3. The darker the color of the cedar the better the flavor.
4. Place the salmon on the soaked plank and season with kosher salt liberally.
5. In a small bowl, mix together the mustard, sriracha, mayonnaise, and brown sugar. Spread evenly over the salmon.
6. In a large deep dish, toss the asparagus and onions in the olive oil and sprinkle with salt.
7. Brush an area on the grill lightly with oil. Place the onions and asparagus on the oiled grates. Cook for about 3-4 minutes turning halfway until lightly charred and just tender. Remove from heat and keep warm. Place the salmon plank on the grill and cover with lid. Cook the salmon for 10-15 mins, or until 130-135 F degrees on a thermometer. Let rest for about 3-4 minutes. Place the plank on a large tray and serve alongside with the grilled vegetables.