Preheat oven to 275 degrees. In a shallow baking pan just large enough to fit salmon in a single layer, spread out fennel slices, half of citrus slices, scatter smashed garlic cloves, then pour olive oil over top.
Nestle the salmon fillet over the top of the citrus, making sure at least the bottom is submerged in oil. Drizzle with a little more oil, season generously with salt and pepper, then scatter remaining citrus slices over top of salmon.
Scatter thyme sprigs around pan, then pop into the oven for 25 – 30 minutes, until the fish flakes easily with a spoon. I like to insert a knife in the center to peek and make sure it’s cooked to my liking – I like it to be firm and pink but not raw in the center.
Remove from oven and use a serving spoon to break it apart a bit. Top with fennel fronds if desired, season with a little more salt and pepper, and eat up!