Cook pasta in a large pot of boiling salted water until al dente. Drain pasta, reserving 1 cup pasta cooking liquid. Meanwhile, heat oil in a large skillet over medium-high. Cook sausage, breaking up with a wooden spoon, until cooked through and browned and crispy in spots, 68 minutes. Using a slotted spoon, transfer to a plate. Add sliced fennel to skillet; season with salt and pepper. Cook, stirring occasionally, until fennel begins to soften, about 5 minutes. Continue to cook, adding 1/4 cup water to skillet as bottom begins to brown (you may need to do this a few times), until soft and caramelized, 1015 minutes more. Add pasta and reserved pasta cooking liquid to skillet and cook, tossing to coat, until liquid has reduced to a creamy sauce, about 4 minutes. Add sausage and fennel fronds, season with salt and pepper, and toss to combine.