Preheat oven to 400 degrees F.

Place the potatoes and garlic cloves in a large pot of salted water. Bring to a boil and cook until the potatoes are fork tender. Drain and add the potatoes and garlic back into the pot. Add the butter and milk and mash with a masher or a hand mixer. Keep the mashed potatoes a little bit chunky, not perfectly smooth. Season the potato mixture with salt and pepper, to taste.

Heat 1 tablespoon olive oil in a skillet over medium heat. Add the onions and cook until browned, about 6 minutes. Stir in the anchovies and cook for 2 more minutes, until anchovies 'melt' into the onions. Remove the onions from the pan and add in the sausage. Cook the sausage until browned and cooked through, about 6 to 8 minutes. Slice the sausage into thin rounds.

On a lightly-floured surface, using a lightly-floured rolling pin, roll out the pizza dough into a 1/4-inch thick round. Transfer the dough to a parchment-lined baking sheet and bake in the oven for 10 minutes.

Top the pizza crust with the onions. Spread the mashed potatoes over the onions and top with sausage slices and Cheddar cheese. Bake in the oven for 20 minutes. Slice and divide between 2 plates. Serve with the arugula salad on the side.

To make the arugula salad: whisk together the lemon juice, olive oil and salt, and pepper, to taste. Toss the arugula, fennel, and celery in the dressing. Divide the salad between 2 plates and top with shaved Parmesan.