For the onions:
Coat a large saute pan with olive oil, add the onions, and season with salt; bring the pan to medium-high heat. Cover and sweat the onions for 15 to 20 minutes, or until soft and beginning to color.
Add the balsamic vinegar and cook, uncovered, for another 20 minutes or until it becomes syrupy. Taste and reseason if needed. Remove from the heat and set aside.
For the pate:
Coat a large saute pan with olive oil and add the anchovies, capers, and garlic. Bring to medium heat and cook, stirring occasionally, until the anchovies dissolve, 3 to 4 minutes.
Add the chicken livers, turn the heat to high, and cook for 5 to 6 minutes, stirring occasionally. Add the wine and cook for 4 to 5 more minutes or until the wine has reduced by half; the mix will still be pretty soupy.
Transfer the mixture to a food processor and puree until smooth. Season with a little salt and loosen with a little olive oil if needed. Schmear each toast with the pate and top with the balsamic onions to serve.