For mushy peas, melt butter in a pan over a medium-low heat. Add onion and garlic. Cook 3 minutes or until softened. Add peas, spinach and 1/3 cup (80ml) water, and cook 3 minutes. Take off heat, add mint, and pulse with a stick blender for 1 minute or until roughly chopped. Season and set aside.
Half fill a large, heavy-based saucepan with oil and heat to 190°C.
Sift flours into a large bowl and stir through fennel seeds. In a small bowl, whisk egg, soda and ice. Just before cooking, gradually whisk the soda mixture into the flour until just combined. It should be a bit lumpy and the consistency of thin pancake batter.
Working in batches, lightly dust each fillet with extra flour, dip in batter, and drop into oil. Deep-fry for 2-3 minutes or until crisp and lightly golden. Use a slotted spoon or tongs to lift out. Drain onto paper towel and season with salt.
Divide fish onto warmed plates. Serve with mushy peas, lemon wedges and salad.