Melt half the butter and oil in each of 2 sauté pans or skillets over medium heat. Add the livers to one pan and the onions and sage to the other. Cook, stirring frequently, until the livers are cooked through but still pink in the center and the onions are tender and translucent, 5 to 8 minutes. Add the Marsala to the livers and cook a few minutes. Using a slotted spoon, transfer half the livers and half the onion mixture to a food processor. Process with pulses until coarsely chopped (not pureed) and transfer to a mixing bowl. Repeat with the remaining livers and onion mixture. Stir the pan juices into the liver mixture and season to taste with salt and pepper. You may want to add a little lemon juice or more Marsala to round out the flavor.
Brush both sides of the bread slices with olive oil, then grill or broil the slices until golden and toasted on both sides. Spread the bread with the warm liver mixture and serve warm or at room temperature.