Heat oil in a skillet over medium heat. Season the chicken with salt and pepper, then place it in the skillet. Cook for 6-8 minutes per side, until golden brown and fully cooked through. Transfer the chicken to a plate and set aside.

In a small bowl, whisk together the cornstarch and a splash of cold water to make a slurry; set aside.

Reduce the heat to medium-low and add the shallot and garlic to the skillet. Saute for about 1 minute, until fragrant. Remove the skillet from heat briefly for 30 seconds to slightly cool the pan (adding the sauce to a pan that’s too hot can cause the yogurt to curdle).

In another bowl, whisk the broth and room-temperature Greek yogurt until smooth. Slowly pour this mixture into the skillet while whisking constantly. Stir in the cornstarch slurry immediately to stabilize the sauce and prevent curdling. Add the Dijon mustard, then return the skillet to low heat and continue whisking gently.

When the sauce is just beginning to thicken, stir in the lemon juice. For an extra smooth texture, you can blend the sauce and return it to the pan.

Return the chicken to the skillet and spoon the sauce over the top. Let it simmer gently, uncovered, for 2-3 minutes, until the chicken is heated through and the sauce has thickened slightly. Serve.