Peel and cut potatoes into 1" dice. Bring a medium pot with potatoes covered by lightly salted water to a boil. Reduce to a simmer and cook until potatoes are fork-tender, 15-18 minutes. Drain in colander and return to pot. Add sour cream and 3/4 the cheddar cheese (reserve remaining for topping potatoes), then mash with a potato masher or fork until smooth. Season with 1/4 tsp. salt. Cover and set aside. While potatoes cook, bread chicken.

Rinse chicken breasts, pat dry, and season both sides with 1 tsp. salt and 1/4 tsp. pepper. Place panko breadcrumbs in a medium mixing bowl and mix with 2 tsp. olive oil. Place liquid egg in a second medium mixing bowl. Dip chicken into egg, shake off excess, then dip and completely coat in panko, using your hand to press breading onto chicken. Place on prepared baking sheet. Repeat with remaining chicken breasts.

Place baking sheet in oven and bake until breading is golden brown and chicken has reached a minimum internal temperature of 165 degrees, 12-15 minutes. Remove from oven, sprinkle each chicken breast with a generous pinch of salt, and allow to rest 2-3 minutes. While chicken cooks, bake potatoes.

Scoop cheddar mashed potatoes into prepared medium casserole dish and sprinkle with remaining cheddar cheese. Bake until cheese is melted and slightly browned, 10 minutes. While potatoes bake, cook peas.

Mince chives. In a small bowl, stir together Dijon mustard and honey. Place a medium pan over medium-high heat. Add 2 Tbsp. water, peas, and 1/2 tsp. olive oil to hot pan and cook until peas are warmed through and bright green, 2 minutes. Season with 1/4 tsp. salt and a pinch of pepper.

Serve cheddar mashed potatoes on a plate and sprinkle with chives. Place peas next to potatoes and chicken in front of potatoes. Place honey mustard in front of chicken or serve on side.