Put diced onion in medium sized bowl. Add Salt, mix and keep at room temperature for about an hour
Heat Olive Oil in skillet over medium heat. Saute onions for 10 minutes
Add Black pepper and half the Thyme. Stir.
Reduce heat to medium low, add red wine/balsamic vinegars. Cook for 10 minutes, stirring occasionally. Mixture should be thick and heavy
Add sugar, lemon juice/zest and stir on medium-high heat. Cook for 15 minutes, stirring occasionally, making sure nothing is sticking to bottom. Should be viscous. Remove from heat, let cool.