This sounds very much like The Crystal Empire by L. Neil Smith, published 1986.
In the setting, instead of killing around 1/3 of the population of Europe, the 1351 epidemic of plague is enhanced by accident and kills about 99% of everyone on Earth. Six hundred plus years later, all memory of the plague time has been lost to the Helvetii, except for the forms of ritual purification and a culture that seems to derive from Scandinavia.
Into this world, Sedrich Sedrichson is born, the son of a blacksmith and berserker, mothered by the female leader of their religion. A natural mechanic and inventor, he quickly runs afoul of the male side of their gender-divided religious leadership, who confiscate his invention of a wind turbine powered paddle boat and forbid him to invent anything else.
Fast forward to his late teens, young Sedrich is inventing things again, but doing it in secret; he once more comes to the attention of the male religious leadership, just as he "reinvents" gunpowder from the questions the religious males always ask when they're hunting for those who would search for forbidden knowledge. In the ensuing quarrel, his father dies of a stroke, he kills the male religious leader's goon with the first iteration of a simple gun, and then, unable to escape the situation, applies their punishment to himself, cutting off his own right hand with his father's sword.
On regaining consciousness, he flees the village, eventually setting up a freehold in the wilderness. Later in the story, he encounters an empire that has been using cross-time psychic viewing, along with spies and agents, to steer the civilization of their continent (what we'd call North America), and are attempting to subvert the other major civilization, the empire of the Muslims (descended from the early Ottomans). They have a weapon in the form of a huge, solar-pumped ruby laser built into an Aztec pyramid, and use mirrors hung from airships to direct the pulses against enemies anywhere short of the horizon (as seen from an airship at altitude, that's a hundred miles or more).