I picked up a pair for $2.69 (there's also a plain version with just roast beef on the slider buns for the same price).
Mighty Minis were offered earlier this year in select participating markets and it looks like they did well enough to be offered nationally.
The star of the show were the crispy onion strings, which were similar to French's but fresh. There's a pronounced, seasoned, caramelized onion flavor to them that you don't quite get with onion rings. Along with the au jus sauce, they really set off the roast beef.
The roast beef was sliced razor thin and tender but lacking texture, which is pretty par for the course for Arby's. It didn't feel as salty as usual, which led me to enjoy it more than usual.
Nutritional Info - Arby's Onion Mighty Minis (184g)
Calories - 550 (from Fat - 270)
Fat - 30g (Saturated Fat - 7g)
Sodium - 1260mg
Carbs - 47g (Sugar - 6g)
Protein - 23g