Brush bread slices with 1 tablespoon of the olive oil.
In a small bowl, whisk together remaining 2 tablespoons olive oil, vinegar, mustard and salt and pepper to taste. Drizzle half over arugula and toss well; set aside remaining dressing for another use. Stir walnuts into arugula mixture.
Spread each slice of bread with 1 tablespoon ricotta, then top with 1 tablespoon arugula mixture. Serve immediately.