In a pan large enough to hold all the fillets in a single layer, melt the butter over high heat.
Season the flounder with salt and pepper and dredge in the flour. When the butter is very hot, add the flounder and cook until browned on one side. Carefully flip the flounder and brown well on the other side. Cook until the fish flakes easily. Remove from the pan and set aside.
Add the lemon juice and parsley to the butter in the pan and set aside.
In another large pan, heat the olive oil over medium-high heat. Add the garlic and sauté until it just starts to brown. Add the chard and raisins and cook, stirring, until the chard is wilted and tender, about 5 minutes. Season with salt and pepper.
Divide the chard and raisins among four plates. Place a fillet on each plate, and pour some of the butter and lemon sauce over each. Serve immediately.