Fill one-third of a large saucepan with water and bring to a simmer. In a large heatproof bowl, toss the strawberry slices with the sugar, cover with plastic wrap, and set over the simmering water. Heat until the strawberries release their juices, about 1 hour. Line a fine-meshed sieve with cheesecloth, place over a bowl, and pour in the strawberry mixture. Strain well (but do not press the berries or the juice will turn cloudy) at room temperature for 2 hours. Reserve the juice, chilled.