1. Preheat oven to 400 degrees.

2. In a medium bowl, mix together onion, oil, 1 tablespoon salt, and pepper. Spread onion mixture in an even layer on a rimmed baking sheet and transfer to oven. Bake until onions are tender and beginning to brown, about 15 minutes. Remove from oven and let cool to room temperature.

3. In a small nonreactive saucepan, mix together vinegar, sugar, and remaining 2 1/2 teaspoons salt; bring to a boil over high heat, stirring occasionally, until sugar has completely dissolved. Remove from heat and stir in red pepper flakes, mustard seeds, chile powder, garlic powder, Chinese five spice, and turmeric.

4. Place cooled onion mixture, fennel stems, cucumber, jalapeno, and bell pepper in two 1-quart glass jars or one 2-quart glass jar; toss to combine. Pour hot vinegar mixture over vegetables, adding water if necessary to cover. Cover jar and transfer to refrigerator; refrigerate at least 24 hours before serving, and up to 2 weeks.