Pat the chicken breast halves dry with paper towels, then season them with garlic powder, salt, and pepper. Coat each piece lightly with flour, shaking off any excess.

Heat 2 tablespoons of olive oil over medium heat in a large skillet. Once hot, add the floured chicken breasts and cook for about 5-7 minutes on each side until golden brown and cooked through. Transfer to a plate and cover to keep warm.

In the same skillet, lower the heat and add 1 tablespoon of butter and 1 tablespoon of olive oil. Once melted, add the chopped onions and sauté for about 2-3 minutes until translucent.

Add the sliced cremini mushrooms to the skillet, seasoning with salt. Cook for about 5-6 minutes until they release moisture and begin to brown. Add the minced garlic and cook for an additional minute until fragrant.

Stir in the Dijon mustard and Worcestershire sauce, mixing well. Pour in the chicken broth, scraping any bits stuck to the skillet.

Bring the mixture to a gentle simmer and cook for about 3-4 minutes. Reduce heat and stir in the sour cream until the sauce is creamy and smooth.

Slice the cooked chicken and return it to the skillet, stirring gently to coat in the creamy sauce. Allow to heat for a couple of minutes.

Taste and adjust seasoning with more salt, pepper, or mustard if desired before serving.