1. Melt butter in a 10- to 12-inch frying pan over medium-high heat. Add onion and stir until limp but not brown, 3 to 5 minutes. Add chicken and broth; cover pan, reduce heat, and simmer gently until chicken is no longer pink in the center (cut to test), about 10 to 12 minutes. With a slotted spoon, transfer chicken to a food processor, leaving broth in pan. Increase heat and boil broth, stirring often, until reduced to about 1/4 cup, 8 to 15 minutes. Scrape liquid and any remaining onions into food processor.
2. Add ricotta cheese, parmesan cheese, egg, nutmeg, and salt to food processor; whirl until mixture is finely ground.
3. Follow steps 1 through 3 under "Wrapping Tips," below, to cut egg roll wrappers, fill with chicken mixture, and roll up cannelloni.
4. In each of two shallow 9- by 13-inch baking dishes, spread about 1/4 cup of the creamy tomato sauce over bottom to coat lightly. Set the cannelloni, seam down, in sauce, spacing equally. Carefully spoon the remaining sauce over cannelloni to coat wrappers well.
5. Slice teleme cheese as thinly as possible and lay slices over cannelloni to cover (if the cheese clumps, press between your fingers to flatten), or scatter fontina over cannelloni.
6. Bake cannelloni, uncovered, in a 375° oven until sauce is bubbling, cheese is melted, and filling is hot in the center, 25 to 35 minutes. Transfer to plates with a wide spatula and spoon some of the sauce around each serving.
1. While cannelloni pasta tubes come in basically one length, egg roll wrappers can be cut to fit different-size pans. For 9- by 13-inch baking dishes, stack the wrappers, a few at a time, and trim them into 4- by 6-inch rectangles, discarding scraps.
2. Lay egg roll wrappers in a single layer on a counter. Spoon about 5 tablespoons chicken filling down the length of a 6-inch edge of each wrapper. With the back of a spoon, a small spatula, or your hands, shape the filling into an even log the length of the wrapper.
3. Roll each wrapper tightly around the filling to enclose, and lay it seam down to hold the roll shut.