Make the pickled fennel and quinoa: Bring vinegar, sugar, 3 tbsp. salt, mustard seeds, peppercorns, thyme, bay leaf, fennel, and 2 cups water to a boil in a 4-qt. saucepan; cook 5 minutes, then remove from heat. Let cool, then strain, discarding liquid, thyme, and bay leaf; transfer to a bowl. Stir in half the oil, salt, and pepper; set pickled fennel aside. Wipe pan clean and add remaining oil; heat over medium-high. Cook chile flakes and shallot until soft, 1–2 minutes. Add kale and salt; cook until wilted, 1–2 minutes. Transfer to a bowl. Add stock to pan; boil. Add quinoa and reduce heat to medium-low; cook, covered, until tender, about 25 minutes. Uncover and stir in kale mixture; set aside.
Make the grouper: Heat oil in a 12" nonstick skillet over medium-high heat. Season grouper with salt and pepper; cook until golden, 5–7 minutes. Flip fillets; add 4 tbsp. butter and the thyme. Cook, basting continuously, until fillets are cooked through, 2–3 minutes; transfer to a plate. Add wine to pan; cook until evaporated, 3–4 minutes. Add juice; reduce by half, 5–6 minutes. Discard thyme; stir in remaining butter, the crab, sumac, salt, and pepper. To serve, divide quinoa between 6 plates; top with grouper and spoon sauce over top. Garnish with pickled fennel, the fronds, and, if you like, microgreens.