Trim the bases of the fennel bulbs along with any woody tops. Save any fronds for later in the recipe. Halve the fennel bulbs and thinly slice. Peel the onions and thinly slice. Peel the garlic and crush or finely chop. Pick the rosemary leaves and finely chop.
Pour 1 tbsp oil into a large pan and bring to a medium heat. Slide in the chopped onions, fennel, garlic and rosemary. Sprinkle in 2 tsp smoked paprika, 1 tbsp ground cumin and a generous pinch of salt and pepper. Fry for 12-15 mins, till the vegetables are softened and slightly caramelised.
While the vegetables cook, scrub and peel the potatoes, and cut them roughly into small 1-2cm chunks.
Slide the potatoes into the pan with the vegetables. Tip in the tin of cherry tomatoes. Crumble in the stock cube and pour over 1ltr boiling water. Return the stew to the boil, then reduce the heat to a simmer and cook for 30 mins, till the potatoes are tender.
When the stew is nearly ready, stir in the spinach leaves. Grate in the lemon zest. Squeeze in half of the lemon juice. Have a taste of the stew and add more salt and pepper or lemon juice if needed.
Ladle the stew into deep bowls and drizzle a little olive oil over the top. Scatter on any reserved fennel fronds and serve.