FOR THE FETA:
2 cups blanched almonds, soaked in water for 12 to 24 hours
1 cup sauerkraut liquid (may substitute a vegan cheese starter/rejuvelac; see headnote)
1/2 teaspoon sea salt
2/3 cup water
2 tablespoons powdered agar-agar (see headnote)
FOR THE BRINE
6 cups water, or more as needed
3/4 cup sea salt or kosher salt
For the feta: Drain and rinse the almonds.
Place them in a high-speed blender (such as a Vitamix), along with the sauerkraut liquid and salt. Puree on HIGH for 1 to 2 minutes, until smooth and no longer grainy; if you don’t have a high-speed blender, it may be easier to blend half the almonds, liquid and salt until smooth, then add in the other half to finish blending. Pour the mixture into a clean container and cover with a plastic, metal or glass lid (or plastic wrap). Let sit at room temperature for 1 to 2 days, making sure you taste it each day, until it begins to get tangy. Keep in mind there is no hard-and-fast rule about how long it needs to culture — your taste buds will have to guide you in determining the right length of time. In warmer weather, it could be just a day, while in cooler weather, it could take 2 days or longer.
Once the cheese is slightly tangy, you can move on to solidifying it. First, prepare the mold for the cheese by lining an 8-inch square pan with cheesecloth. Combine the water and agar in a medium saucepan and whisk well. Cover the pan with a lid and warm through over low heat. Don’t peek for 3 to 4 minutes, then check to see whether it is gently bubbling. At first, if you peek too early, it may look as though it has solidified. However, if you let it cook a few minutes more, it will start to liquefy again and bubble away; the mixture will be a little cloudy and slightly thickened, but still pourable when ready.
Once the agar has fully dissolved, pour in the cultured almond mixture and whisk until fully incorporated. Pour the cheese mixture into the cheesecloth-lined pan. Refrigerate for several hours, until very firm.
For the brine: Whisk together the water and the salt in a large bowl until most of the salt has dissolved. Cut the cheese into 4 equal pieces and place in the brine. Cover and let them sit at room temperature for 8 hours.
To finish, transfer the feta to a storage container and pour the brine over the cheese until it is halfway submerged. Add more plain water to completely cover the cheese and dilute the brine. Cover and store in the refrigerator for up to 3 or 4 months. The flavor vastly improves after the first 3 to 4 weeks.
VARIATION: To make marinated vegan feta, cut 8 ounces of the plain/brined feta (that has been refrigerated for 3 to 4 weeks) into 1-inch cubes, placing them in a clean 16-ounce jar as you work. Add 1 teaspoon herbes de Provence, 1/2 teaspoon dried crushed red pepper flakes, a wide swath of lemon rind and enough olive oil to cover the feta cubes. Cover and refrigerate for 24 hours before serving with bread or crackers.
Ingredients are too variable for a meaningful analysis.