Begin by pressing the firm tofu to remove excess moisture. Place the tofu between two plates and add a heavy object on top for at least 30 minutes.
Once pressed, cut the tofu into thick slices, about 1/2 inch (1.3 cm) each.
In a mixing bowl, combine the lemon juice, apple cider vinegar, olive oil, nutritional yeast, tapioca starch, garlic powder, salt, onion powder, and dried oregano. Whisk until smooth.
Gently coat the tofu slices with the marinade, ensuring each piece is well covered. Allow it to marinate for at least 30 minutes, or longer for a more intense flavor.
Preheat your grill or frying pan over medium heat. If grilling, lightly oil the grill grates to prevent sticking.
Cook the marinated tofu slices for about 4-5 minutes on each side, or until they are golden brown and slightly charred. If frying, ensure they are crispy on the outside.
Remove from heat and let the vegan halloumi cool slightly before serving. Enjoy it warm in salads, sandwiches, or as a standalone snack.