Trim chicken breasts of any visible fat and pat dry with paper towels.
Pound breasts to an even thickness of approximately 3/4 inch using a meat tenderizer.
In a large pot, combine water, kosher salt, garlic, peppercorns, bay leaves, thyme, and rosemary to create the poaching liquid.
Bring poaching liquid to 180°F (just below simmer) - look for small bubbles at the edge of the pot.
Gently lower chicken breasts into the liquid and maintain temperature between 170-180°F for 12-15 minutes.
Remove chicken when internal temperature reaches 160°F and rest for 5 minutes (carryover cooking will bring it to safe 165°F).
While chicken rests, transfer 1/2 cup of the poaching liquid to a small saucepan and reduce by half over medium-high heat.
Whisk lemon juice, Dijon mustard, and chopped fresh herbs into the reduced liquid and simmer for 2-3 minutes until slightly thickened.
Slice rested chicken against the grain into 1/2-inch pieces and arrange on plates.
Drizzle with herb sauce just before serving.