For the pasta: Cook the rigatoni in a large pot of generously salted boiling water until al dente, about 3 minutes less than the package directions. Drain, reserving 1/2 cup of the cooking water.
Heat the olive oil over medium-high heat in a large Dutch oven. Sear the mushrooms, undisturbed, for 3 minutes; stir, and season with 1 teaspoon salt. Sear on the reverse side an additional 2 minutes. Remove the mushrooms from the pan.
Lower the heat to medium and melt the butter in the Dutch oven. Add the garlic and flour, and cook, stirring, for 1 minute. Whisk in the milk and simmer gently until the sauce just thickens, 5 minutes. Remove from the heat and add the Parmesan, Gruyere and 1/2 teaspoon salt. Let cool slightly, then add half of the seared mushrooms, and puree with an immersion blender.
In a large bowl, toss the pasta with the sauce and the remaining half of the mushrooms, adding about 1/4 cup of the reserved pasta water or more if desired. Transfer to a baking dish.
For the gremolata: In a medium bowl, combine the kale, Gruyere, panko, smoked almonds, olive oil and lemon zest.
Scatter the gremolata over the pasta and bake until golden on top and bubbling, about 40 minutes.
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.