Heat oven to 400 degrees. Place the baguette slices on a baking sheet and brush both sides of the bread with the oil.
Bake until golden brown, 4-5 minutes per side. Top as desired. The crostini can be toasted up to 3 days in advance. Keep at room temperature using an airtight container.
Using one batch of this crostini, you can make the following.
For Artichoke and Olive Crostini, toss 1 (14 oz.) can artichoke hearts (rinsed and chopped) with 1/4 cup chopped kalamata olives, 3 T. olive oil, and 1 T. chopped flat-leaf parsley; season with salt and pepper. Dividing evenly, top 24 crostini with the artichoke mixture, then shaved parmesan.
For Smoked Salmon Crostini, combine 4 oz. cream cheese, 1 T. chopped dill, and 1 T. prepared horseradish; season with salt and pepper. Dividing evenly, spread the cream cheese mixture on 24 crostini. Top with 4 oz. smoked salmon and sprinkle with more dill.
For Mushroom and Herb Crostini, cook 12 oz. thinly sliced assorted mushrooms in olive oil in a skillet over medium-high heat until tender, 4-5 minutes. Add 1/2 cup white wine and simmer until evaporated, 3-4 minutes; season with salt and pepper. Dividing evenly, top 24 crostini with the mushrooms and sprinkle with chopped chives.
For Ricotta and Roasted Tomato Crostini, on a rimmed baking sheet, toss 4 cups grape tomatoes with 3 T. olive oil; season with salt and pepper. Roast at 400 degrees until beginning to burst, 20-25 minutes. Dividing evenly, spread 1 cup ricotta on 24 crostini. Top with the tomatoes and sprinkle with thyme.
For Blue Cheese and Fig Crostini, dividing evenly, spread 1/4 cup fig chutney on 24 crostini. Top with 4 oz. thinly sliced blue cheese.
For Peppered Roast Beef, combine 1 T. horseradish with 1/4 cup sour cream. Place a dollop on each crostini, topping with 4 oz. thinly sliced roast beef. Sprinkle freshly cracked pepper over each crostini.
For Caesar Salad, combine 2 cups finely chopped romaine lettuce with 2 T. Caesar salad dressing. Spoon mixture onto crostini and sprinkle with freshly grated parmesan cheese.