Preheat oven to 400°F/200C. Line a baking pan with foil or parchment.
Cut the peppers into large pieces, removing the seeds and veins.
Peel the celeriac, cutting away any crevices that can't be peeled. Cut into 1"/2.5cm cubes
Peel and chop the onion into quarters or eighths. Peel the garlic cloves. Halve the tomatoes.
Place all the vegetables and aromatics on the prepared baking pan.
Drizzle everything with olive oil and sprinkle on the crushed red pepper flakes and smoked paprika.
Bake for 20 minutes, until the celeriac is soft and the tomatoes are gooey and bubbling.
Turn the oven to broil and move the pan closer to the broiler for 5 minutes to lightly char the edges of the peppers and caramelize the tomato juices.
Remove from the oven and allow to cool slightly.
Add all the vegetables into a blender, pouring in any liquid that has settled in the bottom of the pan.
Add 2 cups/470ml broth, 1 tbsp/15ml vinegar, and ½ tsp/3g salt and blend until smooth.
Add more broth to desired consistency. Taste and adjust vinegar and salt as needed. The amount of broth may impact the salt so should be added first.
Pour soup into a pot and heat.
Serve into bowls and garnish with a light drizzle of yogurt, olive oil, cracked pepper and chopped parsley.