Making Smoked Haddock with Buttered Spinach & Mustard Sauce can be achieved with the following steps:

1. Prepare the Fish: In a pan, add the fish stock or water and bring it to a simmer. Place the smoked haddock fillets in the simmering liquid and poach for about 8-10 minutes until cooked through.
2. Cook the Spinach: In another pan, melt half of the butter over medium heat. Add the spinach and sauté until wilted, about 3-4 minutes. Season with salt and pepper, and then set aside.
3. Make the Mustard Sauce: In the same pan used for the spinach, add the remaining butter and olive oil. Once melted, mix in the Dijon and whole grain mustard. Stir continuously.
4. Add Lemon Zest and Juice: Slowly add the lemon zest and juice to the mustard mixture, stirring until well combined. If the sauce is too thick, add a bit of fish stock or water to adjust the consistency.
5. Combine Components: Once the haddock is cooked, carefully remove it from the poaching liquid. Place the haddock on a serving plate, and layer the cooked spinach on top.
6. Drizzle Sauce: Generously pour the mustard sauce over the assembled fish and spinach.
7. Final Touch: Garnish with additional lemon zest or seasonal herbs for an extra pop of flavor.

These simple steps will lead you to an incredible dish that will tantalize your taste buds.