Prep ingredients: Preheat oven to 400 degrees. Zest lemon and squeeze 1 teaspoon juice; cut remaining lemon into wedges. Finely chop shallot and keep half (save rest for own use). Grate Parmesan. Pat chicken dry; place between 2 sheets of plastic wrap. Using a meat mallet or heavy skillet, pound to an even 1⁄2-inch thickness.

Roast broccolini: Trim broccolini stem than halve or quarter spears lengthwise if large. On a baking sheet, mix broccolini with 2 tablespoons olive oil and season with salt and pepper. Roast in oven until well browned and crisp-tender, 10–12 minutes.

Sear chicken: Meanwhile, season chicken with salt and pepper. Melt 1 tablespoon butter with 1 tablespoon oil in a medium skillet over medium-high. When butter foam subsides, add chicken and sear until golden brown on one side, 3–4 minutes (chicken will not be cooked through). Transfer chicken to a plate.

Make sauce: Immediately add chopped shallots to same skillet and cook until softened and browned, 1–2 minutes, scraping up any browned bits. Add 1⁄4 cup water, chicken broth concentrate, and lemon zest. Bring to a simmer. Remove skillet from heat, then stir in all of the sour cream.

Bake chicken: Carefully pour any chicken juices from plate into skillet, then whisk to combine. Transfer chicken to skillet, browned side- up. Sprinkle chicken (not sauce) with panko, then top with Parmesan. Drizzle topping generously with oil. Bake in upper third of oven until sauce is bubbling and chicken is cooked through, about 8 minutes.

Broil chicken & serve: Switch oven to broil. Broil chicken until topping is golden and crisp, 2–3 minutes (watch closely). Reheat broccolini on lower oven rack, if desired, 1–2 minutes. Transfer chicken to plates. Whisk lemon juice into sauce. Spoon sauce onto plates alongside chicken. Serve with broccolini and lemon wedges over squeezing over top, if desired.