Place the bacon into a 5qt Dutch oven in one, even layer. Over medium-low heat, allow the bacon to render until crisp, about 20-25 minutes. Transfer the bacon to a plate lined with paper towels.

Raise the heat to medium and add the red onions to the bacon fat. Cook until tender and caramelized, about 5-6 minutes. Add a splash of water and scrape off brown bits from the bottom. Add the scallions and cook for another minute. Season with salt and transfer to a plate.

Add enough olive oil to the pot to coat the bottom. Add the brussels sprouts and cook until lightly browned and tender. Stir in the lemon zest and juice, season with a pinch of salt, and transfer to a plate.

Add more olive oil and the kale. Cook for a minute until slightly wilted and transfer to a plate. Reserve the pot.

In a food processor, pulse the bread and bacon until coarse breadcrumbs form. Heat a skillet over medium heat, add olive oil, and cook the breadcrumbs until golden and crisp. Season with salt and reserve.

Drop the pasta into a large pot of boiling salted water and cook for 1 minute less than package instructions.

Wipe out the pot and return it to medium heat. Add the butter and let it melt. Whisk in the flour and cook for about 30 seconds. Slowly pour in the heated milk in a few additions, whisking continuously.

Bring the mixture to a simmer, reduce the heat, and cook for about 6 minutes until slightly thickened, stirring often. Stir in the cheddar until smooth.

Drain the pasta and add it to the sauce, along with the onions, brussels sprouts, and kale. Stir well to combine and taste for seasoning. Serve topped with bacon breadcrumbs and chives.