Add onion and olive oil to a large frypan and turn on heat to medium-high. Cook for approximately 5 minutes.
Add fennel and cook 5 more minutes.
Add garlic and fry for 30 seconds and then add the tinned tomatoes, water and parsley. Reduce heat to a simmer and cook for 10 minutes.
Evenly distribute prawns throughout the sauce and continue to simmer until prawns are cooked (approximately 5 minutes).
Mix through baby spinach.
Turn off heat and serve with fennel fronds as garnish.