In a large pot of boiling salted water, cook pasta, stirring occasionally, until al dente according to package directions. Reserve ½ cup pasta water, then drain.
Meanwhile, in a large saucepan over medium-low heat, bring milk to a simmer. Add Boursin and cook, stirring, until melted and well combined, about 3 minutes. Return to a simmer and cook, stirring occasionally, until slightly thickened and glossy, 2 to 3 minutes more; season with ½ tsp. salt.
Add cooked pasta to sauce along with a splash of pasta water. Cook over medium-low heat, gently stirring and adding more pasta water as needed, until sauce coats pasta; season with salt, if needed.
