I want to plot lines for a specific single value from the output in Nsolve (we need to find these nodes from the output ufun, then plot them).
ufun = NDSolveValue[{-Laplacian[u[x, y], {x, y}] == 10, u[x, 0] == 0,
u[x, 1] == 1}, u, {x, 0, 1}, {y, 0, 1}];
ContourPlot[ufun[x, y], {x, 0, 1}, {y, 0, 1},
ColorFunction -> "TemperatureMap",
PlotRange -> MinMax[ufun["ValuesOnGrid"]],
PlotLegends -> BarLegend[Automatic], AspectRatio -> Automatic]
The results something like, the given value is 1.5, the lines are denoted by red