In a small skillet, heat 1 tablespoon of the butter over medium heat until melted. Add sage and cook until fragrant. Remove and discard sage. Remove sage butter from heat and set aside.

Heat oil and 1 tablespoon of the remaining butter in a large saucepan over medium heat. Add onion and cook, stirring occasionally, until onion softens, about 5 minutes. Add rice and cook, stirring, until rice becomes translucent. This process is known as tostatura, or toasting.

Add wine, stirring, until it is mostly absorbed, then add 1 cupful broth and saffron. Simmer, stirring frequently, until broth is almost absorbed. Continue adding broth in ½ cupfuls, stirring often and allowing each addition to mostly evaporate before adding next, until rice is tender but still slightly firm to the bite and mixture is creamy (you will have broth left over).

Stir in remaining 3½ tablespoons butter, reserved sage butter, Parmigiano-Reggiano, and salt to taste. Add an additional cup of broth, stir to combine, and serve “all’onda” (a “wavy” or wet-style risotto) immediately.