Heat the olive oil in a large 12-inch skillet over medium-high heat. Add sliced sausage rounds and cook for 2-3 minutes per side until golden-brown and caramelized. If pan is crowded, cook in batches for better browning. Remove sausage to a plate and set aside.

Lower heat to medium and add butter to the same pan. Once melted, add minced garlic and stir constantly for 30 seconds until fragrant but not browned.

Pour in chicken broth and whole milk, scraping up browned bits from the bottom of the pan. Stir in ranch seasoning mix until dissolved, then bring to a gentle simmer.

Add dry rotini pasta directly into the simmering liquid, stirring to distribute pasta evenly. It's okay if some pieces peek above the surface initially. Cover with lid and cook for 12-15 minutes, stirring every 3-4 minutes, until pasta is al dente and most liquid is absorbed. Mixture should still look slightly saucy, not dry.

Reduce heat to low and gradually stir in shredded white cheddar cheese in 3-4 portions, stirring each addition until melted before adding more. Add spinach and roasted red peppers, stirring for 1-2 minutes until spinach wilts. Return browned sausage to pan, mix everything together. Taste and adjust seasoning with salt and pepper. Let rest 2-3 minutes before serving to allow sauce to thicken slightly. Serve immediately.