Preheat oven to 400°F (200°C).

Bring a large pot of salted water to a boil.

Cut potatoes into 1-inch cubes.

Boil potatoes for 10-15 minutes, or until slightly tender.

Drain potatoes and return to the pot.

Add butter, minced garlic, and thyme sprigs to the potatoes.

Heat over low flame until garlic is fragrant and beginning to brown, then discard the thyme sprigs.

Remove from heat and pour in cream, salt, pepper, pecorino Romano cheese, and blue cheese.

Lightly mash the ingredients together, leaving some lumps if desired.

Grease a 3-quart casserole dish.

Place the potato mixture into the casserole dish.

In a small bowl, combine olive oil, breadcrumbs, Parmesan cheese, rosemary, and parsley.

Spoon the breadcrumb mixture evenly over the potatoes.

Bake uncovered for 20 minutes, or until golden brown and bubbly.