Prepare the sauce by combining the peanut butter, sweetener, lime juice, soy sauce and red pepper flakes in a medium-sized bowl. Adjust seasonings to taste, cover, and set aside.
Toss the tofu in 1T of low-sodium soy sauce or 2T teriyaki sauce and air-fry at 375 degrees or bake on non-stick foil or parchment paper-lined baking sheet at 400 degrees for 15-20 minutes
While the tofu is air-fry/baking, fill a large pot with water, and place it over high heat to boil.
Add the kale first and let it shrink down a bit and become tender (about 5 minutes), then add Udon noodles and boil for about 4-6 minutes, until tender. Depending on your noodles and kale, it may take longer. Check for tenderness of both the noodles and the kale.
Strain the kale and noodles in a colander, and let it sit at least 5 minutes so most of the water is drained out.
Return the noodles and kale to the pot and add the sauce. Over medium heat, warm the ingredients and gently stirring them with a wooden spoon or other mixing utensil. You want the sauce to thicken a bit and stick to the noodles and kale.
Serve immediately, topping each bowl of the kale and noodles with the air-fried or baked tofu and a small amount of chopped, dry-roasted peanuts.