Béchamel:
6 Tbsp. unsalted butter
3 Tbsp. all-purpose flour
3 cups whole milk
1 oz. Parmesan, grated (about 1/2 cup)
Pinch of cayenne pepper
Pinch of ground nutmeg
Kosher salt
Assembly:
2 bunches green Swiss chard
3 Tbsp. extra-virgin olive oil
1 lb. sweet Italian sausage, casings removed
1 lb. whole-milk fresh ricotta
Kosher salt
1 lb. fresh pasta sheets (about 16 (6x4") pieces) or 12 oz. dried jumbo shells (about 30)
2 cups store-bought marinara sauce
Grated Parmesan (for sprinkling)
Béchamel:
Melt butter in a medium saucepan over medium heat. Whisk in flour and cook, whisking occasionally, until mixture smells slightly nutty, about 3 minutes. Whisk in milk, then increase heat to medium-high, and still whisking constantly, bring to a simmer. Reduce heat to maintain a simmer and cook just until thick and no longer floury tasting, 8–10 minutes. Add Parmesan and whisk until melted, then whisk in cayenne and nutmeg; season with salt. Cover béchamel and keep warm until ready to use.
Assembly:
Remove ribs and stems from Swiss chard leaves and thinly slice. Tear leaves into large pieces. Combine oil and sausage with Swiss chard ribs and stems in a large skillet and set over medium-high heat. Cook, breaking up sausage with a heatproof rubber spatula or wooden spoon into small pieces, until sausage is almost cooked through, 5–7 minutes.
Add Swiss chard leaves to skillet a handful at a time, tossing and letting them wilt slightly between additions, until all the leaves have been added. Cook, reducing heat as needed (liquid should be bubbling slightly at this point), until liquid is almost completely evaporated (it’s important to dry out the Swiss chard here so the filling isn’t soggy), about 5 minutes. Transfer to a large bowl. Let cool 5 minutes, then stir in ricotta. Season filling with salt.
If using pasta sheets, cook in a pot of boiling salted water 1 minute. Transfer to a large bowl of cold water. Lay out a row of pasta sheets on a rimmed baking sheet (don’t overlap), then top with a sheet of waxed or parchment paper. Repeat with remaining pasta sheets, placing sheets of paper between each layer.
If using shells, cook in a pot of boiling salted water, stirring occasionally, just until very al dente, about 3 minutes less than package instructions; rinse under cold water to cool quickly.
Preheat oven to 375°F. Spread half of béchamel into a 13x9" baking dish. If using pasta sheets, lay out a sheet on a clean surface with a short end facing you. Spoon a heaping ¼-cupful of filling so it runs all the way down the center. Roll pasta around filling to create a 1 1/4"-diameter tube (the pasta will overlap quite a bit). Repeat with remaining pasta sheets, arranging in baking dish as you go to create 2 rows. If using shells, spoon a heaping tablespoonful of filling into each shell and nestle next to each other to create rows (you’ll have about 5 rows of 6 shells each). Spread remaining béchamel over cannelloni, then marinara sauce. Top with Parmesan.
Bake cannelloni until edges of pasta are lightly browned and sauce is bubbling around edges of pan, 30–40 minutes.