Preheat oven to 350 degrees. Line a rimmed baking sheet with parchment paper.
Cut each breast in half, then place on a cutting board and cover with plastic wrap or wax paper. Beat with a meat mallet until 1/2 inch uniform thickness.
Arrange three shallow bowls on the counter. In the first bowl, combine walnuts, rosemary and panko. In the second, lightly beaten eggs and the third, flour mixed with fine sea salt and pepper.
Lightly dredge each breast halve in flour, then egg and finally walnut mixture. Pat in walnuts to make sure you get a good, even coat. Place in a single layer in prepared baking dishes. Repeat with all chicken.
Bake chicken for approximately 20 minutes. For the last 2 minutes, change to a low broil to get a nice brown on top.
While chicken is cooking, whisk together all ingredients for Lemon Yogurt Sauce in a bowl and refrigerate until ready to serve.
Remove chicken from oven and serve immediately. Sauce can be served on the side or directly on top of the chicken.
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