Preheat oven to 350℉. Lay a sheet of foil over a baking pan. Dry the fish fillets with paper towels and place trout on foil.

Brush Dijon mustard over the flesh side of the fish. Sprinkle liberally with bagel seasoning. Use a second sheet of foil to create an airtight packet, rolling the sides of the foil. Transfer the sheet pan with the closed packet to the oven and bake for about 12 minutes. 

While fish is baking, slice cucumber, tomato and onion. Top with olive oil, juice of half of the lemon, and salt and pepper to taste.

After 12 minutes, remove baking sheet of trout from the oven and carefully open the foil packet (watch out for steam!) Discard top piece of foil and return fish to oven for 6-8 minutes, or until trout reaches desired temperature (145 degrees is well done). Remove from oven. The fish will be tender and juicy with a crunchy, slightly browned top. 

To serve, spread half of your serving platter with tzatziki sauce. Scoop chunks of the trout onto the serving plate. Top with tomato salad and fresh dill. Serve with lemon slices and sourdough toast.