Adjust racks to top and middle positions and preheat oven to 425 degrees. Wash and dry produce. 

Peel and dice potatoes into ½-inch pieces. Place in a large pot with enough salted water to cover by 2 inches. Bring to a boil and cook until easily pierced by a fork, 15-20 minutes.

Reserve ½ cup potato cooking liquid, then drain potatoes and return to pot.

While potatoes cook, trim green beans if necessary. Roughly chop sun-dried tomatoes. Zest and quarter lemon.

Toss green beans on a baking sheet with a drizzle of oil, garlic powder, salt, and pepper. Top with half the Parmesan (you'll use the rest later) and roast on top rack until green beans are browned and tender, 14-16 minutes.

In a small bowl, mix together cream cheese, sun-dried tomatoes, basil paste, remaining Parmesan, salt, and pepper.

Pat chicken* dry with paper towels. Holding your knife parallel to the cutting board, carefully cut a horizontal slit into the center of each cutlet to create a pocket (be careful not to cut all the way through!). Evenly stuff pockets with cheese filling. Season on both sides with salt and pepper.

Heat a large drizzle of oil in a large, preferably nonstick, pan over medium-high heat. Add chicken and cook until golden brown, 2-3 minutes per side.

Remove from heat; transfer chicken to a second baking sheet. (Reserve pan for Step 6.) Bake on middle rack until cooked through, 10-13 minutes.

Return pot with potatoes to low heat. Add half the sour cream and 1 TBSP butter (2 TBSP for 4 servings). Using a potato masher or fork, mash potatoes, adding splashes of reserved potato cooking liquid as needed, until smooth and creamy. Season with salt and pepper.

Once pan used for chicken has cooled slightly, wipe out any large burned bits. Return to medium heat; add lemon zest, stock concentrate, half the mustard, remaining sour cream, ¼ cup water, 1 TBSP butter, juice from half the lemon, and a pinch of sugar (all the mustard, ½ cup water, and 2 TBSP butter for 4 servings). Cook, whisking constantly, until everything is combined and sauce has thickened, 2-3 minutes. TIP: Add splashes of plain water if sauce seems too thick.

Taste and season with salt and pepper.

Divide mashed potatoes, green beans, and stuffed chicken between plates. Top chicken and potatoes with pan sauce; serve with remaining lemon wedges on the side.