Preheat the oven to 375F. Line a baking sheet with parchment.
Place the tempeh into a wide, deep skillet or frying pan. Add the broth and tamari. Bring the liquid to a simmer, reduce the heat to low, and simmer the tempeh crumbles until they've absorbed all of the liquid, about 10 minutes. Remove the skillet from heat.
In a food processor fitted with the S blade, process the walnuts, fennel, oregano, and nutritional yeast till the mixture is a fine meal. Add the crumbled tempeh. Pulse to combine all of the ingredients well. The mixture should be dense and a bit sticky.
Roll the mixture into 1 1/2-inch balls. You should have 22-24 total. Place the balls onto the parchment lined baking sheet. Bake the meatballs for 15 minutes. Flip them over, then continue to bake for another 15-20 minutes, or until they're lightly browned.
If you have the time, allow the meatballs to cool for at least 1-2 hours before using. To simmer the meatballs in marinara sauce, bring 2 1/2 cups marinara sauce to a simmer in a sauce pot. Add the tempeh meatballs and simmer for 5 minutes. Otherwise, serve the tempeh meatballs with pasta, in vegan subs, in grain or pasta bowls, or pop one or two as a savory snack!