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When you flatten a sufficiently fatty ball of meat into a patty on a piping hot grill, it sears the exterior and traps some of the moisture. This is why even simple smash burger recipes turn out juicy. However, if you're looking for an upgrade, consider incorporating a cheap ingredient: canned French onion soup.
The beef broth often used in French onion soup will compliment the meat's flavor, and the onions give the patty a little extra kick. Plus, the added moisture from the stock makes for an extra juicy smash burger. Methods vary, but usually incorporating French onion soup into your smash burger is a simple process that doesn't require much deviation from a standard recipe. A good rule of thumb is to use ¼ cup of French onion soup for every pound of beef alongside any other seasonings you intend to use.
One big mistake everyone makes cooking smash burgers is to flatten the patties before putting them on the grill. Instead, roll the meat into balls and place them directly onto the grill before flattening them. Then, cook your patties for a few minutes per side.
Variations on the French onion smash burger

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You can top your French onion smash burgers with standard condiments like ketchup, mustard, and tomato. However, there are ways to add a little something extra to your creation. You can double down on the onion flavor with sautéed or caramelized onions, for example.
Want to really commit to replicating French onion soup in burger form? Add some sherry or brandy and a bit of minced thyme to your ground beef. Then, when it's time for cheese, try forgoing classics like cheddar and American and instead opt for the chef-approved cheese for French onion soup: gruyere. Creamy with slightly nutty undertones, this is a great companion for your burger's bold, beefy flavor.
There are plenty of other fun variations on the trend. If you're a huge fan of onion flavor, try swapping out a regular bun for an onion bun or bagel. Not a big fan of a burger that drips with each bite? Use French onion soup mix instead of a can. That way, you'll get all the extra flavor without the extra mess.