Heat the oil in a deep saucepan over medium heat. If you have a thermometer, the oil should be around 350°F (175°C). If you don't have a thermometer, you can test if the oil is hot enough by dipping a chopstick or wooden spoon into the oil. If it bubbles, the oil is hot enough.
In a small mixing bowl, whisk together the rice flour, cold sparkling water, salt, and baking soda. At this point, you can add your favorite spices or freshly chopped herbs like cilantro or green onions.
Dip mushrooms, one at a time into the batter and slightly shake it off to remove excess batter. Immediately transfer to the saucepan and fry for 3-4 minutes or until golden brown. I recommend frying no more than 3-4 mushrooms at a time.
Once golden brown, transfer the mushroom tempura to a plate lined with a kitchen paper towel to remove excess oil. Repeat with the remaining mushrooms and serve immediately with the dipping sauce!
Dipping Sauce
In a small bowl, combine the soy sauce, water, maple syrup, lime juice, and chopped chili if using. Taste and adjust saltiness, sweetness, and sourness to your liking.
Notes
Your batter must be very cold. I recommend placing your sparkling water in the freezer for 15-20 minutes before starting, so it's very cold. If you are short on time, you can add an ice cube to the batter. Do not add too much ice, though, or the batter will end up being too thin.
The oil must be hot but not too hot. Let me explain: If the oil is not hot enough, your tempura will be greasy and not as crispy. If the oil is too hot, the mushrooms will turn golden brown too quickly and won't be fully cooked inside. I usually test the oil by dipping a chopstick into it. If it starts bubbling, then the oil is hot enough.
Fry no more than 3-4 mushrooms at a time. If you add too many mushrooms at a time, they might end up sticking to each other. Plus, adding too many mushrooms to the oil will slightly lower the oil's temperature, which you don't want.
Adjust the amount of water. Depending on the brand of rice flour used, you might need to add more water. The batter should be thin and not pasty.
For a different dipping sauce, try this garlicky tahini sauce: Whisk together 1/4 cup tahini, 3 tbsp water, 2 cloves of garlic (minced), 1/8 tsp salt, and 1 tsp lime juice. Taste and adjust the saltiness to your liking, and adjust the water to thin as needed.
No nutritional information for fried recipes as it's too hard to calculate.