Preheat the oven to 250 degrees Fahrenheit (120°C) and place serving bowls or plates inside to warm. Heat an oven-safe skillet over medium-high heat and add the olive oil. Season the salmon fillets generously on both sides with salt and pepper. Once the oil is shimmering, place the fillets flesh-side down and sear without touching them for 4 to 5 minutes — they will release naturally from the pan when the crust is ready, deep golden across the entire surface.
Flip once and sear the flat bottom side for just 30 seconds, then flip back flesh-side up and reduce the heat to medium. Add the butter and let it foam. Tilt the pan slightly and baste the butter continuously over the flesh for about 1 minute.
Lay the lemon slices over the top of each fillet and transfer the skillet to the oven. Roast for 20 to 25 minutes, until an instant-read thermometer inserted into the thickest part reads 125 degrees Fahrenheit (52°C) and the flesh is just opaque and slightly custardy at the center. Start the risotto now.
Pour the stock into a medium saucepan and bring to a gentle simmer over low heat. Keep it warm on a back burner for the duration of the risotto — cold stock added to a hot pot stalls the cooking and throws off the whole rhythm.
In the Dutch oven, warm the olive oil and 2 tablespoons of butter over medium heat. Add the leeks with a generous pinch of salt and cook, stirring occasionally, for 8 to 10 minutes until completely melted, jammy, and just beginning to turn golden at the edges.
Add the garlic and cook for about 1 minute until fragrant. Add the arborio rice and stir to coat every grain in the fat. Toast for about 2 minutes, until the grains look translucent at the edges and you hear a faint crackling sound against the pot. Pour in the wine and stir until fully absorbed.
Add the warm stock one ladleful at a time, stirring frequently and waiting until each addition is almost fully absorbed before adding the next — the bottom of the pot should be just barely visible. Keep a gentle, active bubble throughout. This will take 18 to 22 minutes total.
With about 2 ladles of stock remaining, add the asparagus and continue cooking for 3 to 4 minutes until the spears are bright green and bend without snapping. Stir in the peas with the final ladle of stock and cook for 1 more minute, until just warmed through and still vivid.
Pull the pot off the heat. Add the remaining 2 tablespoons of cold butter, the Parmigiano-Reggiano, lemon zest, and lemon juice and stir vigorously for about a minute until the risotto is glossy and flowing. Pull it slightly looser than you think you need — it will tighten on the plate. Taste and season aggressively with salt and pepper.
When the salmon comes out of the oven, remove the lemon slices and squeeze the reserved lemon half directly into the hot pan. The butter will bubble up immediately — spoon it over each fillet until every piece is glossy and coated.
Pull the warmed bowls from the oven, spoon in the risotto, and set a salmon fillet ontop. Spoon any remaining pan butter over the top, finish with extra Parmigiano-Reggiano, and serve immediately.