Roasted celery soup with fennel

SOURCE This recipe is from inpatskitchen on the Food52 website. We have tweaked the recipe slightly, converted the US to European measurements, and made some of the recipe steps more explicit for the novice cook.
Serves 3–4 as main course, 6-8 as a starter
Time (approx): preparation 20m cooking 45m

8 large celery stalks
1 bulb fresh fennel
3 tbsp olive or vegetable oil
1/2 tsp salt
1/2 tsp ground black pepper
6-7 tsp Seven Acres vegetable stock concentrate*
3 medium potatoes
125ml light cream or milk
2 tsps lemon juice
Salt and pepper to taste
* Or use 3 tsp of good-quality bouillon powder or a stock cube.
Knife with a sharp blade
Plastic or wooden spoon or spatula for stirring
Vegetable peeler
Large roasting tin (min. 22cm x 33cm)
Large saucepan
Blender or food processor
Measuring jug
An oven and a hob or other direct heat source for cooking; Space to chop.
Advance preparation
None required, apart from pre-heating the oven to 177°C / 350°F / Gas Mark 4.
Preparing the Vegetables
Wash celery stalks and cut into 5cm pieces.
Cut fennel bulb into 5cm chunks.
Peel garlic cloves.
Peel potatoes and cut into 4cm cubes.
Cooking the meal
Place celery, fennel and cloves of garlic into the roasting tin. Add salt, pepper and oil and toss to thoroughly coat everything.
Roast vegetables in the pre-heated oven for 40 to 45 minutes, stirring occasionally, until the edges of the vegetables just begin to brown.
While the vegetables are roasting, put 1.5L of water into a large saucepan and stir in vegetable stock concentrate or bouillon. Add potatoes, bring to the boil, then simmer until tender (about 10 to 12 minutes).
When the vegetables are done, add them to the saucepan. Let everything cool a bit.
Puree the soup using the blender; if not using a handheld blender, process in batches and return to the saucepan when in is all done.
Bring soup back to the boil. Stir in the cream or milk (if using) and lemon juice.
Season to taste if needed and garnish each bowl with celery leaves and fennel fronds if you like.
Suggested accompaniment
If eating as a main meal, this needs little other than a good rich brown or soda bread. If you want to push the boat out, you could always spread the bread with some soft goat’s cheese.