Preheat your oven to 190°C/Fan 170°C/Gas 5. Trim and peel the beetroot. Slice into around 8 wedges each. Pop them in a roasting tin.
Crack 2 cardamom pods open with the flat side of your knife. Tip them into a pestle and mortar and roughly crush them. No pestle and mortar? Use a small, sturdy bowl and the end of a rolling pin or jam jar. Sprinkle the crushed cardamom seeds over the beetroot with 1 tsp each black onion seeds and black mustard seeds. Sprinkle with a pinch of salt and pepper and drizzle over 1 tbsp olive oil. Toss to mix. Slide into the oven and roast for 15 mins.
While the beetroot is roasting, peel and slice the red onion into 6-8 wedges.
Break the goat's cheese into a small bowl and mash with a fork. Add the yogurt and stir to mix. Finely grate in the lemon zest and squeeze in the juice from half the lemon. Season with a pinch of salt and pepper. Stir to mix and set aside.
When the beetroot has roasted for 15 mins, take the tin out of the oven. Add the red onion wedges and toss to mix. Slide back into the oven and roast for 25 mins till the beetroot is tender when pierced with a knife.
While the veg roast, squeeze 1 tbsp juice out of the remaining lemon half into a mixing bowl. Add 1 tbsp olive oil and a pinch of salt and pepper. Whisk with a fork to make a dressing. Toss the rocket in the dressing to lightly coat.
Pick the mint leaves off the sprigs and finely slice them. Wrap the tortillas in a loose foil parcel. When the beetroot have roasted for 20 mins (35 mins in total) and are very nearly ready, pop the tortillas in the oven on a shelf below the beetroot. Bake for 5 mins to warm them.
Divide the tortillas between a couple of plates. Pile the roast beetroot and onion on top. Spoon over the goat's cheese and yogurt sauce, garnish with the chopped mint and serve.