Step 1: Preheat your oven to 350°F (175°C).
Step 2: In a large skillet, brown the lean ground beef over medium heat until it's fully cooked and no longer pink. Break it up with a spoon as it cooks. Once browned, drain off any excess fat from the skillet.
Step 3: Stir the taco seasoning mix and the undrained diced tomatoes with green chiles into the browned beef. Let the mixture simmer for about 5 minutes, allowing the flavors to meld and the liquid to slightly reduce.
Step 4: In a large mixing bowl, combine the grated sharp cheddar cheese and the ranch dressing. Stir until thoroughly blended and creamy. Add the seasoned taco meat mixture to the cheese and ranch, then gently fold until all ingredients are well combined. The mixture should be creamy but still easy to scoop.
Step 5: Carefully spoon the taco ranch mixture into each of the frozen phyllo cup shells. There's no need to thaw the shells beforehand; they can go straight from the freezer to being filled.
Step 6: Place the filled phyllo cups on a baking sheet. Bake in the preheated oven for 10-13 minutes if the shells were frozen, or 8-10 minutes if they were thawed. They are ready when the cheese is bubbly and melted, and the phyllo shells are golden and crisp around the edges. Allow them to cool for a few minutes before serving to avoid burning your tongue and for easier handling.