Preheat the oven to 220°C (425°F) with a rack in the centre. Line a large rimmed baking sheet with parchment paper.
Halve the baby potatoes (quarter any large ones) so they are all about 2.5 cm (1 inch). Toss them on the tray with half the oil and half the salt and spread them cut side down.
Roast the potatoes alone for 10 minutes so they get a head start.
Meanwhile, whisk the maple syrup, Dijon and lemon juice for the glaze. Pat the salmon fillets dry with paper towel and season them with the rest of the salt.
Take the tray out, push the potatoes to the edges, add the green beans tossed in the remaining oil, and lay the salmon skin side down in the middle. Brush the glaze generously over the salmon.
Roast for 10 to 12 minutes, until the salmon flakes easily with a fork and reads 63°C (145°F) in the thickest part, and the potatoes are golden and tender when pierced.
Scatter with chopped dill and black pepper and serve straight from the tray. Leftovers keep 3 days in the fridge.
Tips
Thicker fillets need a couple of minutes more; the centre should be just opaque.
Swaps
Low-FODMAP: this recipe already avoids onion and garlic; check the Dijon has no garlic.
Storing and reheating
Keeps 3 days. Reheat gently or eat the salmon cold over salad.