Place the semisweet chocolate and 1 teaspoon of the canola oil in the top of a double boiler over just barely simmering water, stirring frequently and scraping down the sides with a rubber spatula to avoid scorching. When the chocolate is melted, stir in 1/4 teaspoon of the peppermint extract. Pour the melted chocolate into the prepared pan, and spread evenly over the bottom of the pan. Sprinkle half of the crushed peppermints over the chocolate layer. Refrigerate until completely hardened, about 1 hour.