Pickled Brussels sprouts

Name: Pickled Brussels sprouts
Ingredients: 1 1/2 kg Brussels sprouts ((12 cups / 3.3 lbs / about 6 dozen medium-sized)); 1 litre vinegar ((5% acidity or higher. 4 cups / 32 oz)); 200 g onion ((peeled, thinly sliced. 2 cups / 7 oz. 1 large. Measurements after prep.)); 150 g red peppers ((sweet, such as bell or shepherd. Seeded, diced. 1 cup / 5 oz. 1 large. Measurements after prep.)); 2 tablespoons mustard seed; 1 tablespoon celery seed; 1 teaspoon turmeric; 1 teaspoon chile flakes; 400 g sugar ((white. 2 cups / 14 oz))