Place salmon in a baking dish with sides.
Make marinade by combining brown sugar, 1/2 cup pineapple juice from pineapple slices, soy sauce, mustard, garlic, and red pepper flakes.
Save half of marinade for salad dressing - set aside for now.
Pour other half of this marinade over the salmon. Let stand for 10-20 minutes.
Turn over and marinade another 10 minutes.
Heat oven to 400 degrees.
Line a baking sheet with foil and lightly spray with non stick cooking spray.
Place salmon skin side down on the prepared baking sheet and bake for 15-20 minutes, or until salmon flakes easily with a fork.
While the salmon is cooking, grill the pineapple slices over medium heat.
With the remaining marinade, add in vinegar and olive oil.
Place spinach leaves on 4 salad plates.
Arrange grilled pineapple slices, tomatoes, avocados, and cheese crumbles on plate.
Top with cooked salmon.
Drizzle salad dressing over top.