Heat the olive oil in a small saucepan over medium heat. When it shimmers, add the onions and 1 tablespoon salt; cook, stirring occasionally, until the onions are softened, just broken down and are beginning to color, 12 to 15 minutes.
Stir in 4 tablespoons vinegar and the orange juice; cook, stirring occasionally, until the mixture is mostly broken down, thickened and golden brown, 7 to 10 minutes.
Remove from the heat; stir in the remaining 2 tablespoons vinegar and the mustard. Taste and adjust the seasoning.
To assemble, top each crisp or cracker with about 1 teaspoon ricotta, 1/2 teaspoon mustard-onion jam and a slice of fig.