Preheat the oven to 375F. Brush 6 non-stick mini loaf pans with olive oil or line them with parchment paper. Set aside.
In a large non-stick pan, add the onion, celery and carrot, salt and pepper and cook over medium heat until the veggies are softened. Add zucchini and spinach and continue cooking for about a minute, until just wilted. Turn off the heat and reserve.
Cut the beef into 1" cubes and place them in the bowl of your food processor. Process on pulse until the meat reaches the right consistency. You might have to work in 2 or 3 batches, depending on the size of your food processor.
Transfer the ground beef to a large mixing bowl. Add the rest of the ingredients, including the reserved vegetables and mix delicately until very well combined.
Divide that mixture between the reserved loaf pans and bake in the oven for about 30 minutes, until the top is nice and golden brown. Note that if you choose to do a single loaf instead, it might require a little longer cooking time.
Remove meat loaves from oven and set to cool on a cooling rack for about 5 minutes before removing them from the pans.
Serve warm (or cold!) with a side of salad.
*You can also use Dijon, Whole Grain or any other type of mustard, just make sure it’s sugar, alcohol and sulfite free if you want to stay Whole30 compliant.