In a Dutch oven or other heavy pot, melt the butter over medium-high heat. Add the onions, raise the heat to high and cook, stirring frequently for about 1 minute then less often until the onions begin to brown, about 5 minutes.
Reduce the heat to medium and add the sugar, baking soda and 2 Tbs water. Cook stirring occasionally and scraping up the browned bits on the bottom of the pot, until the liquid has evaporated, about 1 minute.
Add another 2 Tbs water and repeat this process until the onions are deep golden brown, about 10 minutes.
Add the vinegar and cook for another 30 seconds. Season to taste with salt and set aside.
In a medium bowl, mash the avocados with the lemon juice and a generous pinch of salt, leaving some chunky pieces.
Divide the avocado among the toast slices and top with the onions and peppers. Sprinkle with a little salt.
Serving Size 2 slices