Set the temperature to 400 °F (204 °C). Lightly grease a 9 × 13 inch baking dish so nothing sticks later.

Rinse and halve the baby potatoes, then drop them into a large zip-top bag. Pour in the vegetable oil, Slap Ya Mama seasoning, garlic powder, black pepper, and red-pepper flakes. Seal the bag and shake until every potato is glistening and well coated. This step builds the flavor foundation.

Place a wide skillet over medium-high heat. Add the sausage coins plus the quarter cup of water. Cook, stirring now and then, until the water evaporates and the sausage develops deep golden edges, roughly 8 minutes. Browning intensifies the smoky taste.

Let the sausage cool for one minute so it does not melt the plastic, then funnel it into the potato bag. Reseal and give another quick shake.

Melt the butter in the same skillet. Add chopped onion and minced garlic. Cook until the onion turns translucent and the garlic smells sweet, about 4 minutes. This creates a savory base that perfumes the whole dish.

Cool the onion mixture for a couple of minutes, then scrape it into the bag. Seal and toss so every bite picks up that buttery garlic flavor.

Tip the drained green beans into the bag. Seal once more and shake gently until beans, potatoes, and sausage look evenly distributed.

Empty the entire bag into the prepared dish, spreading the contents in an even layer. Cover snugly with foil. Foil traps steam so the potatoes cook through without drying out.

Slide the dish onto the center rack and bake for about 40 minutes. The casserole is ready when a fork slips easily into the thickest potato piece.

Remove the foil carefully to avoid steam burns. Let the casserole rest for two minutes to settle, then spoon onto plates and enjoy while piping hot.