Step 1

Season chicken with salt and pepper.

Step 2

Heat olive oil in a large skillet over medium-high heat. Add chicken and cook for 5 minutes.

Step 3

Reduce heat to medium and add onion; cook until translucent and soft, about 3 minutes. Add mushrooms and cook, stirring occasionally, for 4 to 5 minutes. Stir in garlic and sauté until fragrant, about 30 seconds.

Step 4

Pour in chicken broth, wine, and sherry and bring to a boil. Stir in Greek yogurt, 1/4 cup dill, and Dijon mustard until well incorporated. Add butter and stir until melted.

Step 5

Place flour in a small bowl. Add a ladle of hot broth and whisk until smooth and no lumps remain. Pour mixture into the skillet and stir until well incorporated.

Step 6

Reduce heat to low and simmer until the sauce starts to thicken, 3 to 4 minutes. Taste and adjust seasoning and/or add additional dill if necessary.