Wash the baby or Yukon Gold potatoes thoroughly. Halve or quarter them depending on size, then place them in a pot of salted boiling water. Cook for 8–10 minutes until fork-tender, then drain.

Pat the cod fillets dry with a paper towel. In a bowl, season with salt, pepper, lemon juice, and lemon zest. Let marinate.

Heat a skillet over medium-high heat, add olive oil and butter. Sear cod for 3–4 minutes per side until golden brown and flakey. Remove and keep warm.

In the same skillet, lower heat and add minced garlic and chopped shallots. Sauté for 2–3 minutes until fragrant. Add rosemary and cook for another minute.

Pour in the broth and bring to a gentle simmer. Stir in heavy cream and Dijon mustard, simmer for 6–8 minutes until thickened.

Gently fold in the boiled potatoes and cook for 3–4 minutes to absorb flavors.

Return seared cod to the skillet, spoon sauce over the fish, and simmer on low for 2–3 minutes.

Garnish with parsley or chives and serve hot, optionally with crusty bread or a light salad.