Combine garlic salt, Herbes de Provence, flour, sugar, and pepper in a small bowl; set aside.

Cut chicken breasts in half horizontally. Sprinkle both sides with the garlic salt mixture.

In a skillet, melt butter and olive oil over medium heat. Add chicken, cooking for 4-6 minutes until golden, then flip and cook for another minute. Transfer to a plate and cover loosely with foil.

Add shallots to the pan and saut? over medium-low heat for 1-2 minutes until softened. Add garlic and saut? for another 30 seconds.

Add wine and bring to medium-high heat, stirring frequently until most of the wine evaporates.

Add chicken broth and bay leaves, bringing to a steady low boil. Cook uncovered for 8 minutes.

While the broth cooks, combine cream, mustard, and cornstarch until smooth.

(Optional) Strain sauce through a fine mesh strainer to achieve a smoother consistency.

Add cream mixture to the simmering sauce along with any accumulated chicken juices. Stir to combine and bring back to a boil, then reduce to a slow simmer for 2 minutes. Remove bay leaves.

Return chicken to the pan, cover tightly, and turn off heat. Let sit for 5 minutes before serving. Garnish with fresh dill.