Preheat oven to 450 degrees. Line a large rimmed baking sheet with foil. Spray with non-stick cooking spray.
Heat a medium sauce pan to a medium heat. Add oil. Add onion and garlic and sweat for 1-2 minutes. Add ketchup, molasses, apple cider vinegar, paprika, brown sugar, smoked paprika, chipotle chili powder, onion powder, dijon, water, and lemon juice. Stir. Bring to a boil and reduce to a simmer. Simmer until thicken, about 15 minutes. Set aside.
While the sauce simmers, bake the salmon. In a small bowl, whisk paprika, onion powder, smoked paprika, chipotle chili powder, brown sugar and 1/2 teaspoon salt.
Cover salmon with rub. Bake until crispy and cooked through, about 15-20 minutes.
While the salmon bakes, whisk mayo, apple cider vinegar, agave, and sriracha together in a medium. Add Brussels sprouts and cilantro to the bowl, along with remaining salt. Toss with dressing. Season slaw with salt and pepper.
When the salmon comes out of the oven, cover with 1/2 cup of barbecue sauce (or more if you would like). Top with slaw. Divide into portions. OR cover with barbecue sauce, cut into portions and then individually cover with slaw. You can also buy pre-portioned filets of salmon, follow instructions as stated.