Preheat & Prepare: Set the oven to 400°F. Season the chicken tenders generously with salt and pepper. Finely chop the shallot.
Sear the Chicken: Heat 2 tbsp olive oil in a large skillet over medium-high heat. Once hot, add the chicken and cook for about 2-3 minutes per side until golden brown. Transfer to a plate.
Sauté the Mushrooms: Add the remaining 3 tbsp olive oil to the skillet. Once shimmering, add the mushrooms, spreading them in an even layer. Allow them to cook undisturbed for 3 minutes, then stir and repeat. After about 9 minutes, when the mushrooms are browned, add the shallots and cook for another minute. Sprinkle with ¾ tsp salt and freshly ground black pepper.
Toast the Bread: While the mushrooms are cooking, arrange the bread slices on a baking sheet. Drizzle with olive oil and lightly sprinkle with salt. Bake for 10-12 minutes until golden and crispy.
Prepare the Sauce: Stir in the flour and cook for about 30 seconds until no white streaks remain. Pour in the chicken stock, white wine, balsamic vinegar, and brown sugar. Add ½ tsp salt and the thyme sprigs. Simmer for 4-6 minutes, stirring occasionally, until the sauce thickens.
Combine & Simmer: Lower the heat to medium-low and remove the thyme sprigs. Stir in the butter until melted, then return the chicken to the skillet, along with any juices from the plate. Let the chicken simmer in the sauce for another 3-4 minutes until fully cooked.
Serve: Dish out the chicken and mushrooms in shallow bowls, spooning over the rich balsamic sauce. Serve with the toasted bread on the side to soak up the flavorful juices.