Cook pasta according to package directions; drain, reserving 1/2 cup pasta cooking water.
Heat oil in a large high-sided skillet over medium-high. Add sausage and mushrooms and cook about 6 minutes, stirring occasionally, until browned. Remove to a plate; carefully wipe pan clean as it may be hot.
Melt butter in pan over medium; add garlic and cook until it just begins to brown, stirring occasionally, 2 to 3 minutes. Add half-and-half and cream cheese; bring to a simmer, whisking constantly until smooth. Remove from heat and stir in 3/4 cup Parmesan cheese, salt, and pepper until smooth. Add pasta and sausage mixture; toss to coat. Thin sauce with reserved pasta water if necessary.
Serve in shallow bowls topped with pesto, basil, and remaining 1/4 cup cheese.
