Place onions in a small saucepan and cover with water. Bring to a boil over high heat. Remove from heat, strain, and transfer to heat resistant medium bowl. Combine vinegar, sugar, and pinch of salt in now empty saucepan and bring just to a boil, stirring to dissolve sugar. Immediately pour over onions. Let sit until cool.
Mash avocado with lemon juice and garlic. Season with salt and pepper. Top each piece of toast with mashed avocado, pickled onion, jalapenos, beans, and sprouts. Serve immediately.
NOTE: Pickled onions will keep in the refrigerator for up to 2 weeks.
Recipe courtesy of Charlotte Abrams