Put the whole beetroots in the centre of a large piece of foil, along with the garlic and oregano. Sprinkle with pepper and drizzle over the vegetable oil and vinegar. Gather up the foil loosely and fold over at the top to seal it. Place on a baking sheet and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 1–2 hours (depending on how large the beetroot are) until tender.
Unwrap the foil parcel and leave the beetroot to cool before peeling and slicing them. Discard the garlic. Make the vinaigrette. Mix together the balsamic vinegar and mustard in a small bowl. Season with a little salt and pepper. Gradually add the olive oil, whisking constantly, until smooth and well combined. Taste and adjust the seasoning as needed, adding a pinch of sugar to reduce the acidity, if liked, bearing in mind the sweetness of the roasted beetroot. Whisk again until the sugar has dissolved. Alternatively, put all the vinaigrette ingredients in a screw-top jar, seal tightly and shake vigorously until well combined.
Put the spinach in a large bowl, and gently toss together with beetroot and orange. Drizzle over the vinaigrette, sprinkle with pepper and tuck in.