In a medium small mixing bowl, Pyrex liquid measuring cup, or a mason jar that can hold at least a cup of liquid, whisk together the miso and orange juice till smooth. This may take a couple minutes; just be patient and keep whisking until the miso has broken down and you have a creamy, thick liquid. There may still be a few tiny clumps of miso visible, which is OK, but the aim is for the liquid to be as smooth as possible.
Add the lemon juice, olive oil, garlic, salt, and pepper to the miso and orange mixture. Whisk again until smooth and emulsified. Serve or store the vinaigrette in an airtight container in the fridge for up to 1 week.