Cook pasta noodles according to package directions and drain. Then let them rest in a 9" x 13" pan with an inch of water in it to keep them moist until the filling is ready to stuff.
While the pasta water heats up you can start your filling.
Heat the oil in the sauce pan. Add the onions and garlic. Sauté for 3 minutes until soft. Do not brown the garlic. Add the ground sirloin and cook until browned. Stir often. Add the tomatoes. Cover and cook for 10 more minutes over low heat. Stir in the breadcrumbs. Remove from heat. Add the romano, egg and spices. Stir thoroughly. Set aside to cool.
Now the sauce. Melt the butter in a saucepan and stir in the flour. You are making a béchamel sauce. Cook gently over low heat for one minute. Stir often. Remove from heat and whisk in the milk and the buttermilk. Do this slowly while you whisk constantly so you don't end up with lumps. Return to heat and bring to a boil; whisking constantly until thickened and smooth. Season with salt, pepper and a dash of nutmeg. Cover and keep warm off of the heat. Reheat and add a little extra milk if necessary to get the sauce back to smooth and pourable.
In a separate sauté pan, sauté the mushrooms with the butter and oil. Cook on medium just until the mushrooms start to brown on the edges. Stir in tomatoes and set aside until ready to assemble cannelloni.
You now have your pasta, your filling, your sauce and your sauce topper (the mushroom mixture). You are ready for assembly. You can even solicit the help of teenagers or other guests to help with this part.
The trick here is to gently handle the pasta so you don't tear it. Even if you do tear it don't be alarmed. It will still taste great. It just won't hold its shape the same way.
When I stuff cannelloni I told the pasta tube upright in my hand. My hand is cupped like a tulip and the pasta is resting the middle. With a small spoon (coffee spoon) I slide a couple of spoonsful of filling into the manicotti noodle.
Arrange the cylindrical noodles in an oiled 9" x 9" oven proof pan. (9" x 13" for larger crowds. Or even two 9" X 13" pans if necessary). Just lay them next to each other as you fill them. You'll have 3 to 4 stuffed noodles per person depending on appetite sizes.
Spoon the white (béchamel) sauce over the noodles laying in the pan. Scatter the mushroom mixture over the white sauce. Sprinkle with the parmesan.
Bake uncovered at 375 degrees for 20 - 30 minutes. You are really just heating the pasta until it's golden and bubbles.
Enjoy. Cheers! Zola