Preparation

Make the chowchow

• Prepare a medium-high (400°F to 475°F) gas or charcoal grill fire.
• Put the vinegar, honey, celery seed, mustard seed, pepper flakes, and 1 Tbs. water in a small saucepan. Stir gently and bring to a simmer over medium-high heat, about 2 minutes. Remove from the heat and set the pickling liquid aside to cool.
• Oil the grill grate. Grill the corn until evenly charred on all sides, about 8 minutes total, turning when necessary. Remove from the grill. When the corn is cool enough to handle, cut the kernels from the cobs, and transfer to a medium bowl. Add the plums and pickling liquid, and season to taste with salt and pepper. Set the chowchow aside at room temperature, about 1 hour.

Grill the trout

• Cut 2 of the lemons in half lengthwise and then crosswise into 1/4-inch-thick slices. Pat the fish dry. Season the cavity of each fish with 1/2 tsp. salt and 1/4 tsp. pepper, and stuff with a thyme sprig and about 3 lemon slices. Tie the center of the fish three times at even intervals.
• Oil the grill grate. Brush the outside of the fish with oil, and season with salt and pepper. (To ease turning whole stuffed fish and keep their skin intact, put them in a flexible grilling basket.) Grill the trout with the grill covered and the vents open, turning once, until a digital meat thermometer reads 125°F, about 12 minutes total. Remove the fish from the grill. Cut the remaining lemon into wedges.
• To serve, remove the twine and put each trout on a serving plate. Use a slotted spoon to put a large spoonful of the chowchow on top of the fish. Serve with lemon wedges.