Preheat oven to 325°F. Coat a large baking sheet with cooking spray.
Make the Riesling sauce: Stir together chicken stock, Riesling, cream, and shallot in a large skillet; bring to a boil over high. Reduce heat to medium; cook, stirring occasionally, until reduced by half to about 1 cup, 12 to 15 minutes. Reduce heat to low. Whisk in butter, 1 piece at a time, until emulsified; stir in lemon juice, salt, and pepper. Set aside, and keep warm.
Make the cod: Arrange thyme sprigs evenly on prepared baking sheet. Brush cod evenly with olive oil, and sprinkle evenly with salt. Arrange 1 cod fillet, skin side down, on every 2 thyme sprigs. Bake in preheated oven until fish is opaque and flakes easily using a fork and a thermometer inserted into thickest portion of fish registers 125°F to 130°F, about 15 minutes. Let stand for 5 minutes.
Make the endive: Heat oil in a large skillet over medium-high. Add endive, cut sides down, and cook until deeply caramelized, 2 to 3 minutes. Reduce heat to medium. Add sherry vinegar; cook until slightly reduced and vinegar tastes slightly sweet, about 2 minutes. Add butter, and swirl skillet until butter is emulsified; sprinkle with kosher salt.
Divide cod evenly among 4 plates. Arrange endive around cod. Spoon sauce evenly over cod. Garnish with flaky salt, and serve with lemon wedges.
