Roast the sweet potatoes. Preheat the oven to 425°F. Toss the sweet potatoes with olive oil, smoked paprika and ¼ teaspoon salt. Spread on a parchment-lined sheet pan. Roast for 25–35 minutes, tossing halfway through.
Hydrate the flax. Mix the ground flaxseed with 3 tablespoons water. Let stand for 5 minutes to thicken.
Prepare the meatballs. Combine the hydrated flax, breadcrumbs, egg, feta, garlic, parsley, oregano, salt and pepper. Add the ground chicken and mix gently until just combined.
Shape and bake. Form into 16 meatballs and arrange on a separate parchment-lined sheet pan. Bake for 15–20 minutes, until the centers reach 165°F on a food thermometer.
Steam the kale. Steam over simmering water for 4–6 minutes, until tender. Drain well.
Assemble the bowls. Divide the kale and roasted sweet potatoes among four bowls. Add four meatballs per bowl, then the tomatoes, scallions and pickled red onions.
Finish with sauce and crunch. Thin the Yum Yum Sauce with water if desired. Drizzle each bowl with 1 tablespoon sauce and sprinkle with 1 tablespoon crispy shallots just before serving.