Preheat the oven to 450°F.
On a rimmed baking sheet, toss the cauliflower with the butter, lemon slices, salt and pepper. Roast until caramelized, 10-15 minutes.
Pat the trout fillets dry and season with salt and pepper. Place the flour, cornstarch, Old Bay, pepper, celery salt, onion powder, garlic powder, cumin and paprika in a shallow dish and dredge each fish fillet in the flour, dusting off excess flour so you have a thin coating.
Heat the oil in a large skillet over medium heat. When hot, add the trout, skin side down, and cook for 2-3 minutes per side, until golden and cooked through.
In a blender, purée all of the gremolata ingredients. Season to taste.
Toss the fennel, arugula, frisee and little gem lettuce leaves in a bowl with the pine nuts, capers and gremolata.
Divide the salad between plates, along with the cauliflower and trout.