Melt a tablespoon of butter in a skillet or pan over medium heat. Add the onion, a pinch of salt, and sauté until it just begins to caramelize (6-8 minutes). Remove from heat and set aside to cool.
Add the ground meat, almond flour and Parmesan cheese, salt, spices, cooked onion, egg ,mustard, and 2 tablespoons of heavy cream to a large bowl. Mix everything thoroughly together using your hands. Scoop using two spoons, or a cookie baller, form into rounds.
Heat up a touch of avocado oil in your skillet over medium heat and cook the meatballs until brown all over and cooked almost through. You'll want to turn them around a few times so they cook evenly. Transfer to a plate while you make the sauce.
Add the remaining butter to the skillet and cook until it begins to brown (don't clean the meatball remnants as they add the flavor to the sauce!). Add in the broth, and remaining heavy cream to deglaze the pan. Allow to simmer for a couple minutes and make a slurry out of your thickener of choice (i.e. mix it with a couple teaspoons of cold water prior to adding it to avoid clumping). Continue to simmer until the sauce just begins to thicken and taste for seasoning, adding more if desired.
Add the meatballs back into the skillet, simmer for a couple more minutes and serve. Sauce will thicken as it cools.