My husband said these are better than any steakhouse side he has ever had. The caramelized onion flavor is absolutely unreal!

For extra richness, you can swap half of the melted butter for 1/2 cup heavy cream or half-and-half, whisking it together with the French onion soup mix before layering. If you want a little more onion texture, add 1 thinly sliced yellow onion in with the potato layers; it will soften down and blend into the French onion flavor.

To lighten things up slightly, use reduced-sodium French onion soup mix and salted butter, then taste at the end before adding any extra salt. For a different cheese profile, try half Gruyère and half Swiss or provolone for a milder, super-melty finish, or add a small handful of freshly grated Parmesan on top before broiling for a sharper, nutty crust.

If you need to prep ahead, slice the potatoes and toss them with the onion-butter mixture, then layer in the slow cooker insert, cover, and refrigerate up to 8 hours before cooking; just add 15–30 minutes to the cook time if starting cold. These reheat well in the oven at 350°F, covered, for about 20 minutes, so they’re a great make-ahead side for holidays or Sunday meal prep.