For the piccalilli: Toss together the onions, fennel and cauliflower with 2 tablespoons of the salt in a colander and set in the sink until they wilt, about 45 minutes. Rinse thoroughly.
Heat the olive oil in a large saucepan or Dutch oven over medium heat and add the turmeric, stirring until the oil is an even color, about 30 seconds. Add the ginger, mustard seeds, fennel seeds, crushed red pepper, garlic and carrots. Cook, stirring, until fragrant and the vegetables are just tender and wilted, about 2 minutes, and then add 2 1/2 cups of water, the vinegars, brown sugar and remaining salt. Bring to a boil, and then remove from the heat and stir in the cauliflower mixture. Let cool to room temperature. Transfer to a sealed container and refrigerate for 24 hours.
For the stotty cake: Whisk together the water, sugar and yeast in a bowl and set aside until the yeast activates and becomes foamy, about 10 minutes.
Whisk together the flour and salt in a large bowl. Rub the butter into the flour mixture, breaking it up with your fingers until incorporated. Make a well in the center, and then pour the foamy yeast mixture and milk into the well. Gradually incorporate the flour into the liquid mixture using a fork. Turn the dough out onto a lightly-floured surface and knead until very smooth, about 5 minutes.
Put the dough into a lightly-oiled bowl and cover with plastic wrap. Place in a warm area until the dough doubles in size, about 1 hour 30 minutes.
Position a rack in the lower third of the oven. Preheat the oven to 425 degrees F. Punch down the dough and remove it from the bowl. Divide the dough into 4 balls, and then shape each into a 7-inch-diameter 1/4-inch-thick circle using your hands. Place the circles, spaced apart, on a baking sheet and cover with a dish towel. Let the dough rise for about 30 minutes in a warm area.
Prick each circle a few times with the tines of a fork, and then make an indentation in the center with your finger. Bake the cakes until golden brown, 15 to 16 minutes. Transfer to a rack to cool. Serve on a board or plate with the piccalilli and any other accompaniments, as desired.
The piccalilli will keep for 1 week tightly sealed in the refrigerator.