Preheat your oven to 425°F. Line a baking sheet with parchment paper.
In a shallow bowl, combine the buttermilk, mixed fresh herbs, garlic powder, salt, and pepper. Add the chicken breast and let marinate for at least 30 minutes in the refrigerator.
In another bowl, mix the whole wheat flour and whole wheat breadcrumbs.
Remove the chicken from the buttermilk marinade, allowing excess to drip off, then dredge it in the flour and breadcrumb mixture until evenly coated.
Place the coated chicken on a separate baking sheet and bake for 18-20 minutes, or until the chicken is cooked through and has a crispy exterior. For extra crispiness, you can finish under the broiler for 2 minutes.
Meanwhile, cut the sweet potato into wedges. Toss them lightly with a small amount of salt and a drizzle of olive oil if desired (optional within caloric allowance) and place on the prepared baking sheet. Bake for 20-25 minutes, turning halfway through, until tender and lightly crisped on the edges.
Assemble the sandwich by placing a piece of baked chicken on the whole wheat bun, layering with leaf lettuce and a slice of tomato.
Serve the sandwich with a side of baked sweet potato wedges and enjoy your nutrient-packed, crispy herb-buttermilk chicken meal.