Make sure oven rack is set to lower-middle position and preheat oven to 425º.
Prepare a 13 by 9-inch baking sheet with non-stick aluminum foil.
After slicing potatoes, toss them with 1 - 2 tablespoons of your favorite olive oil and the minced garlic.
Season with salt and pepper. I just used my hands to mix this up.
Shingle potatoes onto the baking sheet.
Roast potatoes until edges are brown and just tender, 30-35 minutes, rotating dish halfway through roasting.
When potatoes are almost done, prepare fish.
Pat fish dry with paper towels and season with salt and pepper.
Once edges are brown on potatoes, remove from oven and carefully place one fillet, skinned side down on top of each potato pile.
Top fish with butter pieces, thyme sprigs, and lemon slices.
Roast fish and potatoes until fish flakes apart when gently prodded with a paring knife and registers 140º, about 15-20 minutes.
To serve, slide a spatula underneath potatoes and fillets, gently transferring to individual plates.
I removed the thyme sprigs and lemon slices to serve.
I sliced the remaining half of the lemon to serve at the table.