For the chicken: Combine the breadcrumbs, spinach, Parmesan, lemon zest, salt and pepper in a food processor. Blend until the spinach is finely chopped, then transfer to a medium bowl. Mix together the chicken and buttermilk in another medium bowl. Using a slotted spoon, transfer the chicken to the breadcrumb mixture, allowing any extra buttermilk to drip back into the bowl. Toss until the chicken pieces are coated.
Heat the oil in a 12-inch high-sided skillet over medium-high heat (the oil is hot enough when a pinch of breadcrumbs sizzles when added to the pan). Add one-third of the chicken and fry until golden and crispy, about 3 minutes per side. Drain on paper towels. Repeat with the remaining chicken.
For the sauce: Combine the broth, lemon juice and flour in a small saucepan. Bring to a boil, stirring constantly, then lower the heat and simmer until thick, about 3 minutes. Whisk in the butter until smooth. Stir in the parsley, agave and salt. Keep warm.
Pour the sauce over the chicken or serve alongside as a dipping sauce.