Cook the pasta: Bring a large pot of salted water to a boil. Cook pasta until just al dente.
Drain and rinse under cold water to cool quickly, then shake off excess water.
Cook the bacon: While the pasta cooks, crisp the bacon in a skillet over medium heat. Transfer to a paper towel–lined plate and crumble once cooled.
Make the dressing: In a bowl, whisk together mayonnaise, sour cream, buttermilk, ranch seasoning, Dijon, lemon juice, garlic powder, onion powder, and a pinch of salt and pepper. Adjust thickness with extra buttermilk as needed.
It should coat the pasta easily.
Prep the veggies: Halve the tomatoes, dice the cucumber, and finely chop the red onion. Pat veggies dry if very wet to avoid thinning the dressing.
Combine: In a large mixing bowl, add cooled pasta, tomatoes, cucumber, red onion, cheddar, and most of the bacon. Pour in about 3/4 of the dressing and toss until everything is coated.
Taste and adjust: Add more dressing, salt, and pepper as needed.
Stir in green onions and fresh herbs if using.
Chill: Cover and refrigerate for at least 30–45 minutes so the flavors meld. The pasta will absorb some dressing as it rests.
Finish and serve: Before serving, give it a quick toss and add a splash of buttermilk or a spoonful of dressing if it looks dry. Top with the remaining bacon for extra crunch.