Preheat the oven to 425°F. Spray a large baking sheet with oil.
Trim the ends off the plantains. Score each plantain along two sides deep enough to cut through the skins.
Microwave the plantains until they become soft in the center. For 2 plantains this will take about 6 to 6 1/2 minutes. For 1 plantain, about 3 to 3 1/2 minutes, depending on your microwave. The skins will become black.
Peel the plantains and cut them into 3/4 inch thick slices while hot. You should get about 12 per plantain.
Smash the hot plantains (this needs to be done while hot) with the bottom of a jar, can or something flat. I used a tostonera commonly found in Latin Markets or you can buy a tostonera on Amazon for about $6, but anything flat will work.
Place them on the prepared baking sheet and generously spray with oil, season with salt and bake until golden on the bottom, about 10 to 12 minutes. Turn and bake on the other side until golden and crisp, about 8 minutes. Season with more salt and eat right away, while hot.