Preheat the oven to 350 degrees F. Stir together the mustard, Sriracha and 1/2 teaspoon each salt and pepper, and then rub all over the salmon.
Spread the chia seeds on a plate, and then place what would be the skin-side of the salmon on the seeds, pressing lightly to coat.
Heat the oil in a large heavy skillet over medium-high heat until hot. Sear the salmon in two batches, seeded-side down, until browned around the edges and the seeds are set and form a crust, about 3 minutes per batch. Transfer the salmon, seeded-side up, to a baking sheet. Transfer to the oven and bake until just cooked through, 6 to 8 minutes.
Wipe out the skillet.
While salmon bakes, put the bok choy in the skillet with the soy sauce, maple syrup and 1/4 cup water. Cover, bring the liquid to a simmer and cook until the bok choy is tender, about 8 minutes. Uncover and continue to cook until the liquid forms a glaze, 4 to 6 minutes. Serve the salmon with the bok choy.