Heat oven to 425°F. Cook mashed cauliflower according to package instructions.
In a small sauté pan, heat oil and fry dill sprigs, 15 – 30 seconds. Remove dill from oil and transfer to a sheet of paper towel. Set aside until ready to use.
In a large sauté pan over medium-high heat, melt butter with garlic. Cook garlic for 2 minutes until fragrant. Add fish and lemon zest, cook fish either side 3-4 minutes until lightly golden.
Serve fish and butter sauce over cooked cauliflower mash. Sprinkle with fried dill.