Season the pork with salt and pepper. Arrange the sage along the length of the pork; wrap the pork tightly with the prosciutto.
In a large skillet, heat 1 tbsp. oil over medium-high. Cook the pork, turning occasionally and adding the mushrooms halfway through, until an instant-read thermometer inserted into the center registers 145 degrees , 10 minutes. Season the mushrooms. Place the escarole in a bowl. Top with the pork and mushrooms so the warm juices wilt the greens.
Add the rhubarb and onion to the skillet. Cook over medium until crisp-tender, stirring occasionally, 3 to 5 minutes. Transfer the pork to a cutting board; slice into 16 pieces. Add the rhubarb mixture, frisee, snap peas, vinegar and remaining 1 tbsp. oil to the escarole; season and toss.
On a paper towel, microwave the Parmesan on high until puffed, about 50 seconds. Top the salad with the Parmesan croutons and the pork.