Puree everything except the chicken breasts in a blender or food processor.
Lay the chicken breasts in a baking pan in a single layer, and pour the sauce over it.
Refrigerate for around eight hours. (I only did it for two, and it came out fine.)
Turn the breasts over once during refrigeration.
Bake at 400 for ten minutes, turn them over, and bake for another ten minutes.
Serve garnished with shredded chives, scallions, etc.
The Acid Engine
Lemon juice dominates this marinade through sheer volume, providing three-quarters of a cup of liquid. Citric acid lowers the pH of the meat surface. This chemical shift alters protein structures, creating a firmer outer texture while tenderizing the dense muscle fibers. High acidity also brightens the heavy flavor profile of olive oil and aromatics.
Emulsifying the Marinade
Dry mustard acts as a natural emulsifier when blended with olive oil and lemon juice. Mustard contains mucilage and protein compounds that bridge the gap between oil droplets and aqueous liquids. Pureeing the garlic, onion, oil, pepper, salt, mustard, and lemon juice in a machine forces these ingredients into a stable suspension. Without this blending step, the oil and lemon juice would quickly separate in the pan.
Thermal Timing
Lean chicken breasts dry out rapidly when exposed to prolonged heat. Baking the meat at 400 degrees Fahrenheit for twenty total minutes limits moisture loss. Flipping the breasts at the halfway mark ensures even heat transfer from the pan surface to the top layer. Overcooking breaks down the delicate protein gel formed during marination, resulting in a stringy texture.