Roast Sausage and Fennel with Orange

medium fennel bulbs, fronds reserved, bulbs halved through root end, thinly sliced lengthwise

small red onion, halved through root end, thinly sliced lengthwise

sprigs rosemary

extra-virgin olive oil, divided, plus more for drizzling

Kosher salt, freshly ground pepper

sweet Italian sausages (about 1 1/2 lb. total)

medium navel orange

red wine vinegar

Flaky sea salt