Rinse tenderloin and pat dry. Place into large casserole dish.
Dice potatoes into approximately 1 inch cubes. Set aside.
Slice carrots into 1/2-3/4 inch slices. Set aside.
Using garlic press, crush all 6 cloves of garlic into a large bowl. Add onion, rosemary and oil. (Amount of oil depends on how large you cut vegetable pieces.) Stir well to combine all ingredients evenly.
Spoon approximately 1/4 cup of garlic mixture over tenderloins.
Add vegetables to remaining garlic mixture and stir well, coating all vegetables evenly. Spoon vegetables into casserole dish around tenderloins.
Pour any remaining garlic mixture over tenderloins and vegetables.
Place casserole dish, uncovered into oven. Cook for 30-40 minutes, stirring vegetables occasionally.
The more you stir the vegetables, the crispier they will be. For a softer vegetable, only stir at the 25-30 minute mark.