Cut each potato into eight wedges. Trim and thinly slice green onions on an angle, keeping white and green portions separate. Zest lemon, halve, and juice. Finely mince garlic. Pat chicken breasts dry, and season both sides with a pinch of salt and pepper.
Roast the Potatoes
Toss potatoes on prepared baking sheet with 2 tsp. olive oil, ¼ tsp. salt, and a pinch of pepper. Use your hands to rub oil into potatoes and spread into a single layer. Roast until tender and easily pierced, 18-22 minutes, flipping potatoes once after 10 minutes. Toss cooked potatoes, garlic, half the butter (reserve remaining for spinach), green portions of green onions, 2 tsp. lemon juice, 1tsp. loosely packed lemon zest in a large mixing bowl until butter melts and coats potatoes evenly. While potatoes roast, cook chicken.
Cook the Chicken
Place a medium non-stick pan over medium heat. Add 2 tsp. olive oil and chicken breasts to hot pan. Cook until chicken is browned and reaches a minimum internal temperature of 165 degrees, 3-5 minutes on each side. Remove chicken to a plate and season with ¼ tsp. salt. Reserve pan; no need to wipe clean.
Start the Creamed Spinach
Return pan used to sear chicken to medium-high heat. Add remaining butter, white portions of green onion, and spinach to hot pan. Stir occasionally until spinach is wilted, 1-2 minutes.
Finish the Creamed Spinach
Add flour and stir until absorbed. Add milk and Gouda to pan with spinach. Stirring occasionally until cheese is melted and cream is slightly thickened, 2-4 minutes. Add ¼ tsp. salt.
Finish the Dish
Pour creamed spinach on a plate. Place chicken on top of spinach and surround with potatoes. Garnish potatoes with lemon zest.