Season chicken. Preheat oven to 375°F / 190°C. Place ⅓ cup flour, 1 ½ tsp salt, and ¼ tsp black pepper in a large mixing bowl, then mix. Add chicken thighs, then shake to evenly coat on all sides. Set aside.
Sauté mushrooms. In a large skillet or Dutch oven, heat 2 Tbsp butter and 1 Tbsp olive oil over medium-high heat. Add 7oz sliced mushrooms then let cook undisturbed for 4 minutes, or until a deep golden brown color develops on one side. Stir, then continue cooking another 2-3 minutes. Season with ½ tsp salt, ¼ tsp black pepper, and 1 tsp thyme and cook 1 minute more. Add 2 tsp sherry, then use spatula to scrape bottom of pan and releasee any brown bits. Transfer mushrooms to a bowl, then return pan to stove.
Sear chicken. Heat remaining 2 Tbsp butter and 1 Tbsp olive oil in the same pan over medium-high heat. Add chicken and cook until evenly browned, about 7-8 minutes. Flip chicken, then add chicken stock. Transfer to oven and continue cooking until internal temperature reaches 165°F / 74°C, about 10 minutes.
Make the sauce. Remove pan from oven and carefully transfer chicken to a plate. Add 1 tsp sun-dried tomato garlic and ⅓ cup julienned tomatoes. Bring to a boil, then add 1 cup white wine. Continue cooking for 3-4 minutes until sauce has reduced and thickened.
Finish, then serve. Turn off the heat, then add 2 Tbsp marsala and ¼ cup heavy cream. Stir to combine, taste, then adjust seasoning as needed. Transfer chicken back to the pan. Serve immediately with mushrooms on top. Garnish with ¼ cup basil.