For the ragu: Put the garlic, celery, onions and fennel in a food processor and puree to a coarse paste.
Coat a wide, deep pan with olive oil. Add the pureed veggies, season with salt and bring to medium-high heat. Cook the veggies until all the liquid has evaporated and they begin to stick to the pan - you want to brown the crap out of these guys until crud starts to form on the bottom of the pan. Stir occasionally to scrape up the brown bits, and then let the crud form again. Be patient here and don't rush it, this is where the big flavor develops; it will take up to 30 minutes.
When the lovely brown crud has formed and been scraped down a couple of times, add the tomato paste, stirring to combine. Let it start to brown a little and continue stirring for 2 to 3 minutes. There's not much liquid at this point to keep things from burning, so be careful and move fast. Add the wine, stir to combine and scrape up any remaining brown bits. Cook until about half the wine has evaporated, 4 to 5 minutes.
Add both the spicy and sweet sausage and, using a spoon to break it up, cook until the meat is brown (this is where another round of big brown flavors is formed, so take your time), 10 to 15 minutes.
Add enough water to the pan to cover the meat by about 1/2-inch. Stir to combine well, and then add the bay leaves and thyme bundle. Taste, season with salt and taste again; it's by no means done, but it should taste good. Bring the sauce to a boil (BTB) and reduce to a simmer (RTS). Continue cooking, checking occasionally, for 3 hours, tasting, seasoning and adding more water as needed.
For the pickled onions: In a medium bowl, combine the red wine vinegar with 1/2 cup cold tap water. Stir in the sugar, salt and hot sauce.
Add the sliced onions and let sit for at least 1 hour. To assemble: Preheat the oven to 350 degrees F. On a large platter, spread out a layer of tortilla chips. Spoon a layer of the ragu over the chips. Cover with a layer of Parmigianino and fontina cheeses. Place in the oven until the cheese melts, 5 to 10 minutes.
Remove from the oven, sprinkle a layer of arugula over the top and finish with a large dollop of mascarpone cheese. A gooey cheesy treat. Delish.