Put the shallots, garlic, lemon zest, mustard, and salt and pepper in a bowl or a jar with a lid.
Add the lemon juice and the olive oil, and whisk or shake to combine well. Adjust seasonings as needed and use as desired.
Video
Notes
Marinate boneless skinless chicken breasts (or pork chops) for 4 to 8 hours, marinate bone-in chicken for up to 12 hours. If you are using the marinade for shrimp, marinate those for 30 minutes to 1 hour, and fish for 30 minutes max (more on marinating times here).