Halve pomegranate lengthwise and seed. Working in a bowl of cold water, use your hands to separate seeds from membrane. Membrane will float to the top; discard. Scoop out pomegranate seeds and pat dry.
In a large bowl, whisk together 3 tablespoons lemon juice, mustard, and oil; season with salt and pepper. Toss spinach, sunflower seeds, and pomegranate seeds with dressing in bowl. Transfer to a serving platter.
Halve avocado and remove pit. Scoop out flesh and thinly slice lengthwise; drizzle with remaining lemon juice. Transfer to plate and arrange salad on top. Serve immediately.