Heat the olive oil in a pot over medium heat. Cook the onions until soft. Stir in the garlic, then the porter, vinegar, molasses and dates.
Simmer until the dates have softened and broken down and the beer has reduced, about 10 minutes. Remove from heat and allow to cool for about ten minutes.
Add to a food processor and process until mostly smooth.
Preheat broiler. Cut the baguette into 24, 1-inch slices. Arrange bread slices on a baking sheet. Place under the broiler until golden brown. Flip the slices over and place back under the broiler until golden brown on the other side.
Spread each slice with porter date jam, top with crumbled goat cheese, prosciutto and arugula.