Begin by prepping all ingredients as needed and bringing a large pot of salted water to a rolling boil.
In a large nonstick pan, add 1 tablespoon of butter or olive oil, then add sliced halloumi over medium heat. Cook on one side for about 2-3 minutes, then flip. The seared side should be golden brown. If it isn't golden brown yet, continue flipping until both sides are properly seared. Remove and set aside.
Add pasta to the boiling water, cook until just past al dente, then drain and set aside, reserving at least ¼ cup of pasta water.
Meanwhile, in the same pan (or a new pan if you prefer), then add the sliced cherry tomatoes and sauté on medium heat until they become soft, jammy, and release their juices into the pan, about 3-5 minutes. Add in the minced garlic and sauté until softened and fragrant.
Lower the heat and add the mascarpone cheese with ¼ cup of pasta water, and use a wooden spoon or a whisk to combine the cheese into the water to create a sauce.
Add the cooked and drained pasta into the sauce along with lemon juice and the chiffonaded basil. Stir to combine. Add kosher salt and black pepper to taste, along with onion powder. Stir to combine.
Add halloumi in, off the heat, and toss to combine, or save and just add the halloumi to the top. Once plated, garnish with fresh chopped chives.